diff --git a/src/cmd/go/internal/modload/edit.go b/src/cmd/go/internal/modload/edit.go
index dfc366d..47f236c 100644
--- a/src/cmd/go/internal/modload/edit.go
+++ b/src/cmd/go/internal/modload/edit.go
@@ -190,8 +190,8 @@
 
 // raiseLimitsForUpgrades increases the module versions in maxVersions to the
 // versions that would be needed to allow each of the modules in tryUpgrade
-// (individually) and all of the modules in mustSelect (simultaneously) to be
-// added as roots.
+// (individually or in any combination) and all of the modules in mustSelect
+// (simultaneously) to be added as roots.
 //
 // Versions not present in maxVersion are unrestricted, and it is assumed that
 // they will not be promoted to root requirements (and thus will not contribute
@@ -213,18 +213,42 @@
 		}
 	}
 
-	var eagerUpgrades []module.Version
+	var (
+		eagerUpgrades  []module.Version
+		isLazyRootPath map[string]bool
+	)
 	if depth == eager {
 		eagerUpgrades = tryUpgrade
 	} else {
+		isLazyRootPath = make(map[string]bool, len(maxVersion))
+		for p := range maxVersion {
+			isLazyRootPath[p] = true
+		}
 		for _, m := range tryUpgrade {
+			isLazyRootPath[m.Path] = true
+		}
+		for _, m := range mustSelect {
+			isLazyRootPath[m.Path] = true
+		}
+
+		allowedRoot := map[module.Version]bool{}
+
+		var allowRoot func(m module.Version) error
+		allowRoot = func(m module.Version) error {
+			if allowedRoot[m] {
+				return nil
+			}
+			allowedRoot[m] = true
+
 			if m.Path == Target.Path {
 				// Target is already considered to be higher than any possible m, so we
 				// won't be upgrading to it anyway and there is no point scanning its
 				// dependencies.
-				continue
+				return nil
 			}
 
+			allow(m)
+
 			summary, err := goModSummary(m)
 			if err != nil {
 				return err
@@ -234,13 +258,27 @@
 				// graph, rather than loading the (potentially-overlapping) subgraph for
 				// each upgrade individually.
 				eagerUpgrades = append(eagerUpgrades, m)
-				continue
+				return nil
 			}
-
-			allow(m)
 			for _, r := range summary.require {
-				allow(r)
+				if isLazyRootPath[r.Path] {
+					// r could become a root as the result of an upgrade or downgrade,
+					// in which case its dependencies will not be pruned out.
+					// We need to allow those dependencies to be upgraded too.
+					if err := allowRoot(r); err != nil {
+						return err
+					}
+				} else {
+					// r will not become a root, so its dependencies don't matter.
+					// Allow only r itself.
+					allow(r)
+				}
 			}
+			return nil
+		}
+
+		for _, m := range tryUpgrade {
+			allowRoot(m)
 		}
 	}
 
@@ -269,16 +307,41 @@
 		}
 	}
 
-	if len(mustSelect) > 0 {
-		mustGraph, err := readModGraph(ctx, depth, mustSelect)
+	// Explicitly allow any (transitive) upgrades implied by mustSelect.
+	nextRoots := append([]module.Version(nil), mustSelect...)
+	for nextRoots != nil {
+		module.Sort(nextRoots)
+		rs := newRequirements(depth, nextRoots, nil)
+		nextRoots = nil
+
+		rs, mustGraph, err := expandGraph(ctx, rs)
 		if err != nil {
 			return err
 		}
 
 		for _, r := range mustGraph.BuildList() {
-			// Some module in mustSelect requires r, so we must allow at least r.Version
-			// unless it conflicts with an entry in mustSelect.
+			// Some module in mustSelect requires r, so we must allow at least
+			// r.Version (unless it conflicts with another entry in mustSelect, in
+			// which case we will error out either way).
 			allow(r)
+
+			if isLazyRootPath[r.Path] {
+				if v, ok := rs.rootSelected(r.Path); ok && r.Version == v {
+					// r is already a root, so its requirements are already included in
+					// the build list.
+					continue
+				}
+
+				// The dependencies in mustSelect may upgrade (or downgrade) an existing
+				// root to match r, which will remain as a root. However, since r is not
+				// a root of rs, its dependencies have been pruned out of this build
+				// list. We need to add it back explicitly so that we allow any
+				// transitive upgrades that r will pull in.
+				if nextRoots == nil {
+					nextRoots = rs.rootModules // already capped
+				}
+				nextRoots = append(nextRoots, r)
+			}
 		}
 	}
 
diff --git a/src/cmd/go/testdata/script/mod_get_issue47979.txt b/src/cmd/go/testdata/script/mod_get_issue47979.txt
new file mode 100644
index 0000000..f5d4304
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_issue47979.txt
@@ -0,0 +1,117 @@
+# Regression test for https://golang.org/issue/47979:
+#
+# An argument to 'go get' that results in an upgrade to a different existing
+# root should be allowed, and should not panic the 'go' command.
+
+cp go.mod go.mod.orig
+
+
+# Transitive upgrades from upgraded roots should not prevent
+# 'go get -u' from performing upgrades.
+
+cp go.mod.orig go.mod
+go get -u -d .
+cmp go.mod go.mod.want
+
+
+# 'go get' of a specific version should allow upgrades of
+# every dependency (transitively) required by that version,
+# including dependencies that are pulled into the module
+# graph by upgrading other root requirements
+# (in this case, example.net/indirect).
+
+cp go.mod.orig go.mod
+go get -d example.net/a@v0.2.0
+cmp go.mod go.mod.want
+
+
+-- go.mod --
+module golang.org/issue47979
+
+go 1.17
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0 => ./a2
+	example.net/indirect v0.1.0 => ./indirect1
+	example.net/indirect v0.2.0 => ./indirect2
+	example.net/other v0.1.0 => ./other
+	example.net/other v0.2.0 => ./other
+)
+
+require (
+	example.net/a v0.1.0
+	example.net/other v0.1.0
+)
+
+require example.net/indirect v0.1.0 // indirect
+-- go.mod.want --
+module golang.org/issue47979
+
+go 1.17
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0 => ./a2
+	example.net/indirect v0.1.0 => ./indirect1
+	example.net/indirect v0.2.0 => ./indirect2
+	example.net/other v0.1.0 => ./other
+	example.net/other v0.2.0 => ./other
+)
+
+require (
+	example.net/a v0.2.0
+	example.net/other v0.2.0
+)
+
+require example.net/indirect v0.2.0 // indirect
+-- issue.go --
+package issue
+
+import _ "example.net/a"
+-- useother/useother.go --
+package useother
+
+import _ "example.net/other"
+-- a1/go.mod --
+module example.net/a
+
+go 1.17
+
+require example.net/indirect v0.1.0
+-- a1/a.go --
+package a
+-- a2/go.mod --
+module example.net/a
+
+go 1.17
+
+require example.net/indirect v0.2.0
+-- a2/a.go --
+package a
+
+import "example.net/indirect"
+-- indirect1/go.mod --
+module example.net/indirect
+
+go 1.17
+
+require example.net/other v0.1.0
+-- indirect1/indirect.go --
+package indirect
+-- indirect2/go.mod --
+module example.net/indirect
+
+go 1.17
+
+require example.net/other v0.2.0
+-- indirect2/indirect.go --
+package indirect
+
+import "example.net/other"
+-- other/go.mod --
+module example.net/other
+
+go 1.17
+-- other/other.go --
+package other