Anybody know if I can change the Romeo 1 to a Holosun?

I believe the Holosun footprint is compatible with the Trijicon RMR footprint. Romeo1 has its's own footprint and I am not aware of any other red dot that fits the Romeo1 footprint. That said, there are several shops that will re-mill your slide to accept other red dots. Check out L&M Precision.

Sig Romeo 1 cut isn't compatible with any other 3rd parties. Romeo 1 Pro is compatible with Deltapoint Pro. As Hody905 said, you can look at having someone mill it to further accept a different optic. Unfortunately, that method may leave space in front of or behind the optic you want to put on it. The mounting screws would be the only things keeping it in the same place.

Another option I learned about recently are adapter plates from C&H Precision. I'd assume their P226 Romeo 1 to Holosun plate would work on the P229 but would suggest giving them a call.

http://www.chpws.com/product/s...s-factory-optic-cuts