Hey Whatusername... There were multiple conversion kits for 727s to larger/different engines over the years.
The most popular from a pilot perspective seems to be the VALSAN modification. They replaces the #1 and #3 engines with the engines from a Super80 (JT8D-219), which were significantly bigger, more powerful, etc. In the VALSAN conversion they left the #2 engine alone, so the aircraft actually operated with 2 different variants of engine (if you can call them variants... mostly they are different engines and I'm not sure any parts carry over between the two). The reason for not replacing the center engine was the cost for reworking the structure and the extremely expensive proposition of designing an S-Duct to provide ample air for the much larger -219 engines.
From what I've read, the VALSAN 727s were absolute rocket ships. It turned a great plane into a greater plane!
If you google VALSAN 727 you'll find some good reads about it.