It's either a major deviation or a major spoiler. If it's the former, then I think GRRM's best move would be to just rerelease the Theon I preview with the line about putting his daughter in her throne. That would get the point across without calling D&D out directly, similar to the way he's just talked about missing Jeyne to quietly criticize their Sansa plot.
If it's the latter, which I really don't think it is, then we'll be in a really awkward situation. D&D will take a ton of flak for something that was ultimately GRRM's story choice, unless he's willing to release material showing they're being faithful on this beat.
That said, my impression, knowing where the story stands in the books at this moment, and what D&D's tendencies are as storyteller, is that Mel and Selyse will sacrifice Shireen at Castle Black without Stannis' knowledge or consent, as a response to the Pink Letter, and D&D just feel it's more ~dramatic~ if he makes that decision himself. I almost suspect that they just always assumed this was where the character was going, and that when they found out that it's not, they said fuck it and wanted to do the scene they had been imagining anyway, but I don't want to be quite that ungenerous toward them.
