VM rollClientToCurrentAndCreateRegion(VM oldClient, ClientRegionShortcut shortcut,
      String regionName, String[] hostNames, int[] locatorPorts, boolean subscriptionEnabled) {
    VM rollClient = rollClientToCurrent(oldClient, hostNames, locatorPorts, subscriptionEnabled);
    // recreate region on "rolled" client
    invokeRunnableInVMs(invokeCreateClientRegion(regionName, shortcut), rollClient);
    return rollClient;
  }
This test starts up multiple servers from the current code base and multiple servers from the
old version and executes puts and gets on a new server and old server and verifies that the
results are present. Note that the puts have overlapping region keys just to test new puts and
replaces
geode-core/src/upgradeTest/java/org/apache/geode/internal/cache/rollingupgrade/RollingUpgrade2DUnitTestBase.java