diff --git a/libexec/rbenv-sh-shell b/libexec/rbenv-sh-shell
index f4e0098f..9c885e93 100755
--- a/libexec/rbenv-sh-shell
+++ b/libexec/rbenv-sh-shell
@@ -39,25 +39,50 @@ fi
 if [ "$version" = "--unset" ]; then
   case "$shell" in
   fish )
+    echo "set -e OLD_RBENV_VERSION"
     echo "set -e RBENV_VERSION"
     ;;
   * )
+    echo "unset OLD_RBENV_VERSION"
     echo "unset RBENV_VERSION"
     ;;
   esac
   exit
 fi
 
+if [ "$version" = "-" ]; then
+  if [ -z "$OLD_RBENV_VERSION" ]; then
+    echo "rbenv: OLD_RBENV_VERSION not set" >&2
+    exit 1;
+  fi
+  case "$shell" in
+  fish )
+    rbenv_version=$RBENV_VERSION
+    echo "set -e OLD_RBENV_VERSION \"$rbenv_version\""
+    echo "set -e RBENV_VERSION \"$OLD_RBENV_VERSION\""
+    ;;
+  * )
+    rbenv_version=$RBENV_VERSION
+    echo "export OLD_RBENV_VERSION=\"$rbenv_version\""
+    echo "export RBENV_VERSION=\"$OLD_RBENV_VERSION\""
+    ;;
+  esac
+  exit
+fi
+
 # Make sure the specified version is installed.
 if rbenv-prefix "$version" >/dev/null; then
-  case "$shell" in
-  fish )
-    echo "setenv RBENV_VERSION \"${version}\""
-    ;;
-  * )
-    echo "export RBENV_VERSION=\"${version}\""
-    ;;
-  esac
+  if [ "$version" != "$RBENV_VERSION" ]; then
+    case "$shell" in
+    fish )
+      echo "setenv RBENV_VERSION \"${version}\""
+      ;;
+    * )
+      echo "export OLD_RBENV_VERSION=\"$RBENV_VERSION\""
+      echo "export RBENV_VERSION=\"${version}\""
+      ;;
+    esac
+  fi
 else
   echo "false"
   exit 1
diff --git a/test/shell.bats b/test/shell.bats
index 0f776745..3c7313c1 100644
--- a/test/shell.bats
+++ b/test/shell.bats
@@ -22,12 +22,18 @@ load test_helper
 
 @test "shell unset" {
   RBENV_SHELL=bash run rbenv-sh-shell --unset
-  assert_success "unset RBENV_VERSION"
+  assert_output <<OUT
+unset OLD_RBENV_VERSION
+unset RBENV_VERSION
+OUT
 }
 
 @test "shell unset (fish)" {
   RBENV_SHELL=fish run rbenv-sh-shell --unset
-  assert_success "set -e RBENV_VERSION"
+  assert_output <<OUT
+set -e OLD_RBENV_VERSION
+set -e RBENV_VERSION
+OUT
 }
 
 @test "shell change invalid version" {
@@ -42,7 +48,52 @@ SH
 @test "shell change version" {
   mkdir -p "${RBENV_ROOT}/versions/1.2.3"
   RBENV_SHELL=bash run rbenv-sh-shell 1.2.3
-  assert_success 'export RBENV_VERSION="1.2.3"'
+  assert_output <<OUT
+export OLD_RBENV_VERSION=""
+export RBENV_VERSION="1.2.3"
+OUT
+}
+
+@test "shell change version pushes away previous OLD_RBENV_VERSION" {
+  mkdir -p "${RBENV_ROOT}/versions/1.2.3"
+  mkdir -p "${RBENV_ROOT}/versions/1.2.4"
+  mkdir -p "${RBENV_ROOT}/versions/1.2.5"
+  export OLD_RBENV_VERSION="1.2.3"
+  export RBENV_VERSION="1.2.4"
+  RBENV_SHELL=bash run rbenv-sh-shell 1.2.5
+  assert_output <<OUT
+export OLD_RBENV_VERSION="1.2.4"
+export RBENV_VERSION="1.2.5"
+OUT
+}
+
+@test "shell change version to the same version does not lose OLD_RBENV_VERSION" {
+  mkdir -p "${RBENV_ROOT}/versions/1.2.3"
+  mkdir -p "${RBENV_ROOT}/versions/1.2.4"
+  export OLD_RBENV_VERSION="1.2.3"
+  export RBENV_VERSION="1.2.4"
+  RBENV_SHELL=bash run rbenv-sh-shell 1.2.4
+  assert_output ''
+}
+
+@test "shell change version to - swaps old and new versions" {
+  mkdir -p "${RBENV_ROOT}/versions/1.2.3"
+  mkdir -p "${RBENV_ROOT}/versions/1.2.4"
+  export OLD_RBENV_VERSION="1.2.3"
+  export RBENV_VERSION="1.2.4"
+  RBENV_SHELL=bash run rbenv-sh-shell -
+  assert_output <<OUT
+export OLD_RBENV_VERSION="1.2.4"
+export RBENV_VERSION="1.2.3"
+OUT
+}
+
+@test "shell change version to - with no previous is an error" {
+  mkdir -p "${RBENV_ROOT}/versions/1.2.3"
+  RBENV_SHELL=bash run rbenv-sh-shell -
+  assert_failure <<OUT
+rbenv: OLD_RBENV_VERSION not set
+OUT
 }
 
 @test "shell change version (fish)" {