I have extended the wires on a couple servos so far. Did this on my 4 Star 120 because the aileron servos were so far away from the exiting hole on the wing, and on my son's MOJO because that is the only way that it can be built. Both have been working so far.
The MOJO instructions shows disconnecting the servo plug, soldering on a length of wire, feeding the wire inside a carbon fiber tube via slit that is cut in the side of the tube, and then re-installing a plug. Worked pretty good, so far.

Oh, and the plane flew just fine, until I dumped the second landing and bent the gear. My fault, but easy fix.