I am pretty certain the 23 model has not been made for several, several, several years...so, either it isn't a 23, but a later 275/280 or whatever numerological model name they went with 2 years ago...or it IS a 23, and it's fairly older than 2 years old.
Even the Yamaha parts site notes YAS 23 discontinued in 2011....
As far as 'upgrade'....well, as most folks consider modern keywork 'superior' to traditional/vintage mechanics, I suppose one can argue it'd be an upgrade.
(Parts are not an issue, really....techs do just fine with finding parts for old Bundys and it isn't like the original parts would ever just walk off or be damaged beyond repair in 99% of cases).
I would say this:
The 23 (or whatever Yama it is) is gonna feel less...'substantial'...than your Bundy, which is just a robustly-made horn. The Yama will feel significantly lighter and the keywork/action, while slicker for MOST players...will also feel a lighter and a bit less, well, substantial.
The Yama will not sound as full and rich as the Bundy does.
So compute those factors in...only you can decide if it is an upgrade. You say "it may PLAY better"
Exactly what that MEANS can be a subjective thing, if you get my drift....
Does it 'play' better because it is in better, serviced condition ? Does it play better because, both horns being in equally good tack, you find the keywork quicker to navigate around ? Etc, etc....
As I said, 'default opinion' would say that the Yamaha's 'modern ergos' are a 'step up'....I think most sax players would say that. I am just saying that there may be certain attributes of the Yama which do not seem a step up.....so you may 'get' something, but depending upon your personal leanings as a player/musician, you may also 'lose' something....
What you're considering is more a lateral move than an upgrade. Bundy's were based on old Buescher designs and you might actually find more flavor in such a horn than a Yamaha.
This would be the reasonable alternative opinion to the current 'default opinion', yes, and it has some merit....