Like most of our developmental projects, the first mention of them often comes long before an actual product, and I'd guess that less than half of the possibilities mentioned here actually come to pass. Usually there is an upgrade kit to bring an older version up to the new standard. For example SEX and Paramount have recently been improved, and the upgrade for Paramount came out before the revised basic Paramount. Doc is working on the transformer replacement for SEX, which is the sonically significant part of the revision.
Here's the Crack situation in a nutshell, at least the technical part. We tried a modification that increased the current through the output tube. It seemed to improve the sound a bit, so Doc B took that modified version to RMAF. However, the increased current means increased heat under the chassis, so Doc will run it for a few months to see if the extra heat causes any new problems, such as reliability issues. If not, the mod (or a variant of it) would likely become an upgrade kit, and the amp would likely incorporate that the next time it is revised. It's a major pain in the backside to re-write a manual, so we usually wait for enough changes to accumulate before jumping to a revised design.
Probably that doesn't answer your question, but it does give you more information to help make your own choice.
