We try to get a head start on that, at least for major versions which tend to take the longest to support - depending on our capacity. However, some things cannot be done in advance until the final version is released and we know the full extend of changes.
It's a difficult balance on our end because the sooner we start the less time it takes to release a new version but at the same time the more actual time it requires as the compiler keeps changing and we have to redo stuff. For example, it more or less doesn't make sense for us to do anything during beta releases. The earliest we can start is around RC1, RC2 but even that still involves a lot of extra work as we need to retest everything again with the final version.
In this specific case, the fact that a .20 update was released before a major conference that we attend is not great for us as we loose a week being on the conference + we have to deal with so many other things in the weeks before such a conference.