I have tried to do an upgrade of an existing enclosure in the past and it didn't work well. I don't think trying to swap woofers is a good option and swapping amps probably isn't a good option either. This appears to be a passive radiator design which doesn't lend itself to swapping woofers. The existing amp likely has filters to protect the woofer or boost output at certain frequencies. If you swap amps you could kill your woofer.
What kind of upgrade are you after? Lower extension? Higher output? Both?
If you are after lower extension, you could likely redesign a cabinet using using the same driver and amp.
My guess is that you want more output and lower extension. I would buy a new, larger sub and build an enclosure for it. You could probably get by with the amp you have and upgrade later when you can afford it.