quick question about changing hair on models

[Deka_Night]

hey whats up i wanted to do a simple mod of replacing jimmy's hair with jey's i mean i swaped the model file put it all back together the model loads and everytjhing is cool but his hair is like wigging out and on the post match his hair takes up the whole screen is there something im missing or maybe doing wrong i havent attempted to mod a model before so i'm a bit noobish