I've not tried this, although i have repaired punctures in tubs with good success, but apparently this is the way ...
make sure you use a BUTYL tube..
So you dont have to undo all the stitching, you can undo a small amount around the valve area, and cut the old tube in half.
Do the same with your new tube and temporarily fasten the ends together, so that when you pull the old tube out, it draws the new one through.
Once its in, unjoin, then fasten the 2 ends of the new tube together with vulcanizing solution. There should be enough give in a tube to allow reasonable overlap. Make sure it holds pressure, then sew back up.
Reglue the basetape using copydex and voila! (apparently
Ive seen a step by step webguide with pics somewhere, but cant find right now..