That plan is if fine if you just stick with the stock dihedral at first. Now if you want to talk dihedral on a conventional trainer to make it more aerobatic for down the line forget just flattening the wing, lets talk anhedral. Using about the same anhedral as it formerly used dihedral will impart a very neutral roll response much like a pattern airplane and its that 'stays where you put it' feeling that is so prized by the pattern pilot.

Also the airplane will be more stabile than it would be if you had just flattened the wing. Just remember I definately do not recommend this for any one new as the achilles heels is the difficulty in maintaining orientation with the unusual configiguration. This is just one example of the time honored propensity of the modelers tendency to 'kit bash'.

Just don,t do it now!