So I got a poor, sad little Sheertrimmer. I have only ever re-haired one pony, a custom Sweet Scoops. The best advice I found for re-hairing a dream beauty was to cut the hair down as far as possible, then stick the new hair in. I tied the hair in knots, rather than just wrapping it around a finger, and poked it in with the re-hairing tool. This way it stays in, even with gentle pulling. Now she is happy again!
(Don't know how I ended up with two of the same pic, I'm new to photobucket, but wanted to share!)
http://s1272.photobucket.com/user/Saskia-/library/