Good luck swapping the fluid; good move. Check the released fluid for any metal bits, either iron of aluminium (or even aluminum
As Jayson mentioned, normally when hot, because of increased tolerances, but actually failures can go either way, as a result of differential expansion, so hot or cold might produce issues. (For example, if your roof was set up on a really hot day, it would work then, but might be worse when cold. Most cars would be set up on a cold day, and so be worse when hot)
Across the front of the roof is the locking cylinder; if the locks are overtight, or unlubricated, or whatever, then this cylinder may not be able to push the latches when cold. Try tapping the roof with the heel of your hand to try jarring the latches to undo. Next stage is to pull the little cover in the roof beside the dome light, and put an Allen key (hex key) into the socket there, and you may see this turn when opening the roof; chances are it doesn't. You could help it, and you can then work it manually to feel the sorts of loads your latches are causing. If that is the issue, and once released, the rest of the roof works OK, then you can rework the latches, and use the manual function to see when you've got the action nice and smooth/easy.