I am not a lawyer and don't have much experience in Wills and Estates, so you should take this with a grain of salt.
If I remember correctly, you can amend a Will by adding a Codicil, which is a document containing the changes that you want to make to the original Will.
It sounds to me as if the name of your proposed Executor is the only thing you want to change, but if you are changing one thing, that's as good a time as any to review your situation and make any other changes you may desire.
I think your best and "cleanest" option is to do a completely new Will and make it clear that it is your intention that it replace the former one.