It has been my experience that you can always upgrade after final payment, without being penalized (but don't quote me on that). I've been told that Disney won't penalize you for spending MORE money!! lol
As for having booked with a discount code, that won't matter. The only thing that is discounted is the room (passes if it's a 5 Plus 2 package). But, the add-ons are not discounted, so the Silver Plan (or whatever) would be the same additional amount regardless of how our original package was booked.
I figured that it would be along those lines by it being more money. I guess if I called and had the silver plan added that I would just have to pay by CC right then since the final payment would have already been made.
We've added days (and people!) after final payment before with no difficulties. We did indeed have to pay the full difference immediately via credit card. As has been said, I don't think Disney will say no if you offer them more money.
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