Hi DYI:
You cannot install an offset flange into the old flange...
To install an offset flange you will need about 6 inches between the top of the 3" PVC elbow and the finish floor. You will need to cut the pipe above the elbow and then prime/cement the offset flange so the flange itself sits on top of the finish floor. You will also need to plan on opening up the hole in the floor abit to acommodate the offset in the fitting.
A typical offset flange will offset the piping 1.5".
Let me know if you have more questions...
MARK