Although it's not necessarily recommended you can have as many different builds of QLab on your computer at once, as you like. You just need to rename them before you put a new build into the applications folder.
So you could have QLab3122 and QLab3118 in your application folder quite happily. You can even run them simultaneously, though again I wouldn't recommend doing that either.
3.1.18 is a solid build, even though there are some issues that are fixed in subsequent versions. . I have several tours on that build which have been entirely trouble free.
3.1.19, 3.1.20 and and 3.1.21 are best avoided, for reasons which are obvious if you read the change logs (Available under the QLab menu in each build)
Mic