Can a pilot amend a flight plan in-flight? Which information pilot can change in-flight?
For example:

Altitude. Ask to climb higher?
V to Z. VFR rules to IFR (VFR to IFR)?
Anything and everything (well, maybe not the aircraft type...). A flightplan is just that - a plan. The pilot's intentions can change during flight for any number of reasons. Just inform ATC of your new intentions, request a new clearance as appropriate, and ATC will accommodate the request if possible. Happens all the time.