Thanks for the info Ben.

Just to be clear on a few things....

The existing cell (Jandy aquapure) unscrews pretty easily (not shown in pic). I just need to re-plum the new unions since the new cell is larger (evidently Jandy doesn't make the same cell anymore. The new model is a three port and my existing one is a two port).

The main issue I'm having is that the existing plumbing doesn't allow for much modification. All the fittings are right up against each other. So, if there's an elbow, it's connected to another elbow with no gap in between! Very frustrating. I'm guessing that's what you mean by one-shot install. So, I have to take a few steps back before going forward. The hard part is the valve. I can work around the rest. I'll have to take a second look at the valve to see if it's threaded or not. I was hoping to be able to unscrew it, as it looks like part of the existing PVC is screwed in (see pic).

Unless, is it possible to remove the existing unions? I'm assuming not, since everything is glued up. I'm an avid DIYer, but I'll admit this is a bit out of my league.

I'm not going to get too crazy with it. If I think I'm going to get in trouble I'll definitely take your advice and call a pro.