The front module is easily switched out. You can have a 450, 350, or keep the Rex. Call Dive Lab and they can help you get it done. It's best to have a Certified Technician to do this work. A Tech. will have the right tools and torque specs. Over torque is a big problem when it comes to hats. Don't monkey on a hat unless you have the proper tools and are trained to do so.
Let me guess, you want to switch out because of the noise problem? I'm working on it and should have a prototype for testing soon.
The 450 is a great reg. but it doesn't like the mud because of the small diaphragm...sooo, if you do a lot jetting and such, you might not like it.
Because of its size and material composition, it doesn't freeze up as quick as the 350 can...sooo, if you're doing a lot of cold water, you might like it better.