Hi,
Sorry for raising the theme again, but I think too - there will be Metallica!
And it'll be for both HF and GMM. That's the very logical reason for HF to move to the next weekend. I do not believe that it's just for Foo Fighters. To host Metallica for both festivals, one should move. With ability to catch FF, it's a very clever move for HF. Also, that may explain HF wish to sell tickets just after the festival - need cash for booking confirmations.
That also explains the (delay) announcements. HF made an unexpected announcement (usually full lineup recent years) to highlight some of the groups and mostly FF, as if it is announced with double Metallica, it won't be that hyped, as it's now (see on HF forum or FB/Insta). For GMM on other hand make sense to start with double M announcement to make it really big. If it was a plain line up in term of headliners they can already start with JP, A7X, etc.
And about the M dates fixed, message on their site clear states other appearances possible, but not new own dates:
Double M make sense for both festivals to differ from other this year with limited headliners. And it's already confirmed that they can do a festival like this last year with Download. For HF it also make sense to re-book M after 2022, as it'll be a new and unique experience with 2 sets with different setlists and demand for M was huge there.