Hmmm, best study aeronautical engineering before messing with wings. Extending wings will significantly change spar loads, drag, stall speed, Vne, and importantly the pitching moment of the wing which can result in reduced or insufficient elevator authority at various speeds. Roll rate would reduce and rudder size may no longer be appropriate as differential aileron drag on outer wings is higher and requires greater rudder force to balance (note large rudders on gliders). So the risks are: damaging or breaking the spar, not having enough power from existing motor to have sufficient climb rate to safely depart(due to increased drag), structural damage from unknowingly exceeding a now reduced Vne, and not having elevator authority(usually as speed increases), yaw instability plus maybe more.
Shortening wings can have similar and unexpected side effects, surprisingly it can increase drag for a given airfoil as a higher angle of attack is required to generate sufficient lift. This will bring the wing further up the L/D curve into an unsafe zone- as you noted increased stall speed. Increased cruise speed would only occur if you have sufficient HP from the engine as power requirements increase exponentially with speed.
Do lots of study or hire a professional- I have studied this extensively as I am designing a similar change. I am not a qualified engineer and there may be additional issues not noted. Tread carefully.