Is the reason you want to convert it to internal power adjustment so that you can claim it is not adjustable over 12fpe?
If so, I wouldn't bother. If you read the anti-tamper thread, then anicdotal evidence suggests that if a testing house wants your gun to shoot over 12fpe, they will think nothing of stripping it down and winding up the hammer spring as far as it will go, whether it's got an internal or external power adjuster.
I'd suggest the best way would be to fit the smaller transfer port that limits the mamimum power the gun will achieve.
The gun won't shoot as efficiently, but you will have piece of mind.