Maximum Upgrade Potential

You don’t. Because, even if you get the very best of today (I am an Audio Research fan). By the time you break it in and enjoy it for a few years, there will be a new model that is a step up.

Practically speaking. While if you love a certain companies sound. Then five years will get you incremental improvement with the next model… but ten years nets you a really big improvement. I find ten years… actually with any component is a really big difference worthy of investing in.

I owned Pass designed amps for nearly 40 years. And over thst time ten years was a huge difference. I recently heard a couple of them and the improvement in rhythm and pace over the last ten years has been remarkable.

FWIW, I think that for most 'ears' it is more about 'different' than it is about 'improvement'. Given a certain level of quality to begin with (high) IMHO most of what the manufacturers are doing with new products is tweaking a product and, perhaps, just moving the sound in a different direction. Yesterdays 'romantic' became todays resolution/clarity/dynamic range, but not, I think, because of any fundamental changes in design so much as popular expectation. In lesser products new iterations are not necessarily so constrained.  :-)

At over 20 years it don’t owe you much.  My tastes changed over the past 20 years, and if it’s in the budget I’d side with upgrading to another amp depending on your tastes.  There are some great amps out there from the likes of Octave, Audio Hungary, Audio Research, VAC, Unison Research, etc. so just read reviews and give yourself a well-deserved gift after 20+ years if something looks really interesting.  Prima Luna is good for sure, but there are higher levels of performance if you (and your budget) are up for it.  Tweaks and changing tubes will only get you so far.  Enjoy the journey!

I think, you answered your own question - you are ready to upgrade the amp. Cost is always a consideration, of course. I too would not be engaging in endless tube rolling after 20 years of owning a piece.

Without knowing anything about your taste, system and budget, it is hard to suggest any brands, but you didn't ask, anyway.