I have an older Energy 12" subwoofer, it has a 360 watt amp (720 peak) but the thing is over ten years old now, what I was wondering about doing is replacing the driver with something like an SVS plus 12.3 or something along those lines.
I am not very versed in subwoofer mods but can do the mod if it is feasible.
Can anyone tell me if this a) will work b) if it's a bad idea c)recommend a replacement driver (or confirm the 12.3 plus is good)

First, you can't buy the woofer from SVS unless you're a customer. Second, it is a bad idea since the driver is optimized in a large enclosure. Third, it probably won't fit.

This is a common question asked in the DIY section. First you have to find a driver that will work in your existing enclosure. That may take a while since you have to model them in WinISD or Unibox. And the modeling is not correct since you do not know anything about the existing amp's filters or boost. That can be measured but will require additional hardware and software. By this time you could have built a sub using one of the hundreds of proven designs in the DIY section.

Short answer - putting NASCAR tires on your Pinto will not give you race car performance. The driver is just part of the system designed to work together.

You can get drivers similar to the SVS 12.2 from Torrent Audio for $90 if you decide to go DIY.

-Robert

Another option might be to work the other way round...

Measure the interior volume of the box and the port diameter and length, then find the driver parameters that will provide the response you are looking for. Finally, look for a driver with those parameters. This won't be easy - it will probably require many iterations and much searching of driver catalogs, and the result may not be much better than what the Mirage engineers did 10+ years ago. TANSTAAFL! :p