They all work with the current version.
If you have BiaB 2023 everything will work with your MegaPak version; they just show up as available now.
The question is which route is most cost effective?
A lot of us agree that the Ultrapak is the way to go so you have all the base styles & RTs.
Only thing you would be missing is the Xtra Paks that were offered one time previously, and many have noted that most (not all) of those get included in the next year release.

So in the long haul, the UltraPak 'ultimately' saves money as an upgrade VS buying all the new stuff every year. Plus you get the improvements/fixes on some styles that you already have and the new program improvements .. just a thought. Who knows? Some of next years RTs may require the latest version to take full advantage of features, in which case you would be ahead of the game.

Not sure of your budget, but if you can swing it, many users here recommend doing it that way.
Just a thought.
Some years buying the base program (Pro) and upgrading it to UltraPak actually saved money instead of buying the UltraPak in the first place .. but not always.

I do not work here, but the benefits are still awesome
Make your sound your own!