I spoke with a sailmaker about converting a Tornado pinhead to a squaretop a couple of years ago. He did not want to do it, and he tought it was a bad idea.
A friend asked Ullman sails Sicily the same, and they said sure. Turned out that they did not want/could not do it either, and sold him a nice repaired square top instead (Nothing wrong with Ullman Sicily, gave a really good price).

If I was to try this (I am _not_ a sailmaker), I would have buildt a new top section for the sail instead of just adding cloth.
I see several problems with this, one is knowing how much broadseaming is buildt into the old sail and how much draft the luff-curve makes, and making the two match in the new top.
Old and new cloth have different properties, so expect some trouble there as well.

For an example of how bad a conversion/repair can be, look at the link below. These guys had a new leach installed, and it's not backwinding..
http://www.sailinganarchy.com/forums/index.php?act=Attach&type=post&id=362479

Could be a fun little experiment to convert an old mainsail during the winter, without expecting to much from it..