I have never seen anyone build on Red Sea Reefer that include changing the original piping inside the overflow box. Which Reefer you have? I think if you change all the piping including the ones inside the overflow box to 1" you will end up with too strong of flow, IMO. I think it will be more complicated too. Anyway, it is your call and you can decide how much work and risk you are willing to undertake. I am not sure how that will play out. Also, you may end up voiding the warranty. By changing only the piping below the tank, you can always convert it back to the original if you wish to.
Like I said, most of the builds on forums that I have seen, they didn't touch the piping inside the overflow box.
Good luck and looking forward to see your tank.
This screenshot is attached from this person reefer build:
Red Sea Reefer water return mod
At the top of the water return pipe there is a pinky sized pipe that forms an elbow and connects the eyeball return to the pipe. The water return pipe is 28mm O/D. The elbow is approx. 10mm O/D. WHY? I heard the XXL tanks are larger but didn't confirm, and I don't believe the return pipe would...
www.reef2reef.com
This is the return neck that I'm speaking about. No so much the plumbing that leads up to it, just the 90degree here is 5/8 on my tank (which is a XL200)