I am a cynic.... I would allow the model to move up.. allowed by the spell.... then make the roll.... if failed, they are in combat, but cannot strike because as they get close they realize how beautiful their opponent is.. (the magic takes effect)..... otherwise, it is a FREE do over..... and that is unfair and not in the spirit of the game....
"I am going to move up and attack your model...." (makes roll).... "oh, never mid.... do over.. I am going to shoot him instead....."..... oh, never mind... I'll just shoot or attack this other guy....
Allowing them to change their course of action I feel lessens the true effect of this spell.... if you roll for the attack.... and thus the spell, your action should be lost if you fail it.... even for shooting... You go to shoot, make your roll..... (fail).... not only do you NOT shoot, you spend the action admiring your would be foe.... perhaps, maybe... you can ask them out on a date?
Just my 2 cents..... I don't think it is too harsh.. the free do over just doesn't sit well with me... once committed.... you are, well.. committed.... you move up, roll to see if you can attack and if you fail.. you lose your action and are now in melee....