Mummy was running on the same computer systems it used upon opening in 2004. As far as I understand it, they basically ripped out the "brains" of the ride and replaced it with a modern system, which is a big part of why there were so many hiccups as it came back online. That's in addition to whatever other building work was necessary.
I maintain it was a disappointment that they didn't take the opportunity to rethink the first Imhotep encounter (which has looked embarrassing for over a decade at least) or plus other areas of the ride. We got the welcome refreshes of the turntable transition CGI, but I believe that was it.