I am a long-time Bryston owner.
Because Bryston 4B amplifiers have been in continuous production since 1978 a helpful hint in discussions is to always include the specific generation of the 4B amplifier being discussed.
A 4B amplifier that was new in 1997 is the 4B ST which was first introduced in 1996 and remained in production until about 2002 when it was supplanted by the 4B SST. 1997 4B ST price: $2,470. The price of the current model 4B3 is $7,495.
I own a 4B ST purchased new in 2000. It has performed superbly with no failures these past 23 years. I also own the Bryston BP 25 preamp purchased with the amp and with similar performance.
Last year I considered (1) renewing my 4B ST via Bryston's Back To The Future program in which Bryston restores and recertifies any of its earlier amps to their original factory specifications and provides a new three-year warranty; or (2) buying a new 4B3. A description of the BTTF program is online at Bryston's website: BRYSTON_VINTAGE_MEMO.pdf
Bryston is one of the few manufacturers to test, burn-in, and provide individual performance specifications with a detailed certificate for every product that they make, and then to guarantee that performance for 20 years, a standard unmatched by any other amplifier manufacturer. You should check for an original performance certificate with yours. If missing, you can find the original performance specifications for your unit online.
Because I am in my 70's I decided that simply renewing my current unit is the more circumspect course. I did that and am completely satisfied with the results. My renewed unit sounds no different than it did before, meaning completely transparent to the source. But it is nice knowing that the internals are trustworthy for at least the next three years and likely long beyond my need. And I don't know how they did it but somehow it looks better than new.
There are different costs for the renewal service ranging from $1 - 2k depending upon the options selected. This compares well with the alternatives that you might contemplate.
There is one recent review online directly comparing the performance of a 4B ST (not renewed) to the performance of the new 4B3. It may be found here: Bryston Audio 4B³ Power Amplifier | REVIEW - Part-Time Audiophile (parttimeaudiophile.com)
The review is purely subjective with only the most basic specifications for the 4B3 provided. Still, it's something for your consideration that might be useful.
Your situation is interesting and the options promising regardless of the path selected.