As I remember, the Rocket UFW-12 was a sealed sub. You may have to do nothing in all honesty. I would first implement the new 12 inch, make measurements, before adding wood blocks to reduce the acoustic volume. Sealed subs are highly forgiving.
A larger box gives you a lower Qtc and deeper bass extension (lower F6 and F10) than a smaller box. It may sound tighter/cleaner, but that strongly depends on room placement and quality of the woofer you are replacing it with.
If it were me, I would do nothing and just replace the woofer.
If it’s ported, ignore the above, it will need a redesign.
Best,
Anand.