The contents of Golden Chests can be changed retroactively by picking up/dropping Gilded Key. This can even be observed if Isaac has Guppy's Eye, allowing him to see what the chest would have if he did(n't) have the key and choose which payout he wants.
Mega Chests will also have their contents modified if they didn't contain items.
The difference in contents between the two chest payouts are independent from each other; regardless of if the original chest had an item, card, trinket, or pickups, the modified chest can have any combination of them.
Modified chests are still unable to spawn pickups alongside items.
Gilded Key does not retroactively change chests that have already spawned into Golden Chests, or change their contents.
Golden Chests in The Chest will still spawn items.