You have to edit the sketch in question and then break the sides (by deleting them and re-dimensioning them from the original) and the rest. On the other hand, if you use a plan in the assembly, it will be complicated but it is doable.
If you want to do it properly; You should center the sketch in relation to the original and change the plans if you used them in the assembly. But the part will no longer be constrained and constraints will have to be created to freeze the part.