I did an oil change on the lowers of my Reba 29SL at the weekend and I realise now I should have done the Motion Control oil at the same time, but as I didn’t am I right in thinking there is no need to remove the lowers to do it?
Also, is 5wt oil the only type to use in the Motion control? I have some 7.5wt, would that do a similar job?

No need to remove the lowers again.
You can remove the Mo Co unit from the top. Just undo the 22mm cap, gentle wiggle the Mo Co out, turn the fork upside down and let oil drain out. Pump the fork a few times and clean everything with some isopro. alch. and a clean rag on a dowel.
You could use 7.5wt but it will firm up the damping. If you don’t like the feel you now know it is easy to change. Make sure that you lightly lubricate the seals on the Mo Co when you gently wiggle it back in.