Usually it can be done during an overnight possession Saturday night. That might involve the last train or two being replaced by a bus, but that's not uncommon for the B&H▸ line anyway.
Although many bridges have been raised, quite a few have new decks placed on the old abutments, and some are brand spanking new. In the case of many of them they've just been slid/lifted into place within an overnight possession.
You are probably spot on II. It does impress me how much can be done overnight - a recent example was the old footbridge at Thatcham station which from memory was removed on a Friday night. It's absence was most noticeable when I drove past on the Saturday morning!