You can't unilaterally decide to move to a Permit. The line is that if it can hold a CofA then it will and so long as there is a Type Certificate holder for the aircraft, then there is no chance of a move.
You should never have an uncertified mod by an uncertified engineer to a CofA aircraft - or if you do, then don't publish the fact!!!
Which type of aircraft is it you have in mind?