CG doesn't change unless you change the size or location of a flying surface.

I think you're making a good decision to convert to tail-dragger. They weigh less, are easier to build (and maintain) and have less drag.

Good advice about throttling up slowly and also about taxiing around a bit to feel it out. Be sure to put an air-filter over the carb so you don't suck a lot of dirt into the engine. A piece of ladies hose rubber banded on will work.