Test patches have been released as official before, no reason it couldn't happen again. At the moment, the changes aren't really large enough to warrant a new letter, as that kind of implies a certain level of updates. Having said that, I think T to U was mostly bug fixes but there may have been more to it.
Ultimately, we'll get these compatible updates officially now, either labelled as X9 or Y, and incompatible updates a bit later (either labelled as Y or Z). I'm sure there are better things to worry about than naming conventions.