Some are easy & some are work, but it can be done for sure & you will probably like the result. One of the first things that I do in a trainer hop-up is to get rid of all dihedral. They don't fly like trainers after that, but they are still easy to fly (& they still look like trainers, so ya can fool some folks with your trainer-flying ability).[8D]

If your wing is joined by a steel rod, as is the case with some of the Hobbico RTF's, it can be very easy -- simply bend the rod until the wings lie flat & glue balsa wedges into the gap -- presto flat wing.

If the wing has a wooden joiner, you will have to cut it appart & put in a flat wing joiner, plus the obvious repairs & fit-outs.