I really doubt there is an if about it. Has there ever been the heir (1st in line), male I mean as females don't, who wasn't named Prince of Wales, one who took the throne? I could be wrong, correct me if I am, but the oldest son though not automatically, does become Prince of wales by appointment. It may take some time, and he may be Duke of Cornwall and Cambridge for a time, but unless his father dies shortly after the queen, I doubt it is an If.
The reason for suggesting an 'if' is Welsh nationalism. Do the Welsh want another English prince to be The Prince of Wales?
I do think Charles will seek the views of the Welsh Assembly when the time comes to get the views of the Welsh people. If the Welsh people object to another English Prince of Wales then I don't think William will be created PoW but if the Welsh people are happy for an English Prince to have that title again then he will be created Prince of Wales.
I doubt Charles would go to them. Prince of Wales in an honorary title, its not like he'd be appointing a governor or something. Scotland is very Nationalistic, and yet royals are given a Scottish title, and an Irish one as well. Will is the Earl of Strathearn. I don't see the tradition of POW ending any time soon IMO.