I want to up-grade from a 55G to a 75G reef.....help

Currently I have 3 maxijet 1200,
Unless they are modded you'll want more flow, at least 1875gph

Seaclone skimmer, rated for 100G
I've not heard much good about this skimmer but if you're happy with it on the 55g and think it will work on a 75 to your satisfaction it is worth trying

my sump is part fuge, 40G

I have an 200watt heater,
I'd get a second 200 watt.

my return pump is a Catalina 1800,
not familiar with it.

I have a RO/Di set-up, and I'm dosing Ca, Mg, trace elements,
Are you testing and they are low? Unless you're testing and there is a problem with keeping the numbers acceptable for clams, sps, etc. I'd not dose.

I have 75lbs of LR and 50lbs of live sand,
I'd personally get some more sand. Especially if you'll be keeping anything that likes to burrow. If you have dense rock I'd add some more rock. If you have porous rock you should be fine.

I have Nova Extreme T-5 Fixtures w/Lunar Lights that is 216watts

Hmm actually I made a few suggestions on upgrading a few things. On the lights, I didn't comment because it really depends on what you want to keep.

On another note, depending on how long your tank has been set up you may want to go for complete new substrate. Niko or someone else mentioned something along these lines when I was setting mine up that unless you want to go through a horrendous washing cycle of the sand that its best to just start with fresh due to the contaminants that are contained in the sand and the release of those contaminants when moving it over to a different tank.. At least I think thats what was said, but I can't find the conversation now when I want it..