Regarding the refit of previous ships in a class, I think it all just boils down to if it's deemed worthwhile/necessary by higher ups. While older ships could probably be retired in favor of brand new improved models of the same family, that may not be deemed as a good option. Heavily upgrading older vessels will take up a lot of money and requires in depth modifications, however in the end it will result in a higher number of capable hulls being able to be deployed. So it's a question if it's worthwhile to rather replace older ships with newer ones, or upgrading older ships to at least somewhat compare to the newest ships in class in certain aspects. Being able to retain several older ships on top of additional new ones. So it's, as usual, weighing up the costs and theorized benefits.
Of course this happens all the time. I just don't think it happens very often, if at all to MFRs, due to both the expense and the need to upgrade all the associated systems. For example, I don't think the USN is ever going to upgrade any Burke Flight IIAs with SPY-6 GaN radars and ditch their SPY-1D GaAs radars; and they're PESAs on top of that. It's the major updates where alot of things are getting redesigned anyway, where expensive kit like the MFR and its ecosystem gets upgraded, if at all. Of note, Burke Flight I, Flight II, and Flight IIAs all used the same SPY-1D design.
Regarding the Zumwalt, it's the result of an aimless post-cold war USN. Where obsolete ideas where pushed because times obviously never changed. They're very lucky that the Arleigh-Burkes have proven to be adaptable, numerous and to be an effective backbone. Otherwise failures like the Zumwalts and LCS would have hurt them much more than they did anyway. However they served as a perfect example for me, together with the Flight III Arleigh-Burkes, where very costly, in-depth modifications and improvements can be done to a ship class if it's deemed worthwhile.
I would say for the Zumwalt it's actually the opposite. The USN had all the confidence of Francis Fukuyama and felt they would now rule the high seas forever and ever with great power competition firmly in the dustbin of history, and thus the Zumwalt and LCS classes were spawned, intended primarily for littoral missions with minimal attention paid to high end warfare. In retrospect this was delusional thinking of course, but in the meantime the ships were built at top dollar costs. Now the Zumwalt is being squeezed for lemonade, the LCS is being rejected like lepers on the street even though Congress keeps trying to not let it happen, and the Arleigh Burke hull form is getting ever last ounce of potential wrung out of it. They are making yet another white elephant out of the Constellation class, and want to build even more white elephants like the SSN(X) and DDG(X).
