Used to be, they were pretty flexible. Recently, however, they've been forcing folks to cancel and rebook, and if the discount days were used up, it was tough luck.
That said, DD and DS just switched some AP discount days by a week or so, and they gave them no hassle at all.
I have applied a PIN code to an existing reservation in the past. If anything, have 2 reservations and then cancel the one you don't want (if that is possible).
If the code covers his dates and there is still availability under the code for his dates then he can use it. He may have to switch room types depending on what's available with the code (which can be different than GP availability). I honestly thought they did away with actual PIN codes because the only Disney Destinations mail I've gotten since 2013 was for special offers already on the website. Good to know there still around!
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