Its a pretty simple process. Basically, there is a lip that the sub mounts too that is recessed in slightly so that the sub sits flush with the box. Depending on the replacement box you buy, You will need to cut that lip off, or maybey trim it slightly smaller.
Also, some sub boxes have a pillar that pokes up from the back of the box. If your new sub is deeper than the 8", then you will need to chop that off and seal it.
Here are a few tips though:
1. If you are just upgrading the sub, and not the amp, you will not see much gains. Mostly sound quality. You can honestly just snag a better quality 8" and the results will be very similar with less work
2. If you are upgrading the sub and the amp, then you will quickly find that the SVT box isnt the greatest. If you really want some good thump and clear sound, then you will eventually want to upgrade to a custom box.