It's been many years since this happened to my trousers - these days it's the other way...
The method I have seen done is to rip (cut out) the stitches from crotch to rear waistband and remake the seam as required.
If you don't have the skills or confidence to do this many small dry-cleaners have tame repair artistes on call - for a price. Try yellow pages - dressmaking.
Good luck...
Peccavi, May 2010
