Shortening mainsail?

[trecksail]

Hello all,
Just wondering if anyone has shortened a mainsail? Can it be done at the head, by a few feet, increasing the roach, or would it have to be done at the foot and thereby decreasing the length of the foot? The lenght of the foot fits very well on my boom, so I would rather cut some of the head if possible.

Thanks and have a great night,
Joe

[brak]

I had a genoa shortened. They cut it off at the head and just slightly modified the leech to curve into new top. Pretty easy job, apparently.