What worked for me, is going to the queue I wanted the deck to be legal for, finding my deck amongst the ones it's illegal for, then double clicking the deck there to enter the editor. Upon entering it prompts me to change the deck's format (or clone and change it)

UPDATE! that actually worked!
strange is that if you open the deck list, the banned cards of the previous format are still showing highlighterd in red, but you can play your deck and it will show up if you select the new saved format

so the work around is:
- decks menu
- deck details (bottom of the screen)
- change the deck format, and then change the decks name, that will save the new format

as gercod#86759 mention, going to the decks menu, then deck details you the drop down menu shows normally, but it does not save the format
strange is that you can change the name of the deck and that´s saved lol

When you are in the deck itself and go to the details the 'type' pull down menu seems to behind a frame. When you are in your deck overview you can select (not open) the deck and go to 'details'. Now you van change the decktype. But it seems it doesnt save the change :(

Bought the new lorwyn brawl decks but they're stuck on standard brawl

Same issue

Companions are also not available to switch but I suppose that might be a different bug though it seems associated with the same UI issue.

Same issue! (PC/Steam)

me too .

me too, before i couldnt go back to the main deck builder page once i opened the stats menu

I am seeing this same problem.

Same issue for me on a mac

Same issue on Windows even after repairing the game files on version 2026.54.40.10849.1221064

Thank you おりぅ, it work !
But I don't want to do that, I want the game to work as he should.

Same issue for me on Windows client.
Instead of entering the deck edit screen, go to the deck details from the list. After changing the format there, enter the deck edit screen and restart the Arena. By selecting “Resume Editing” to the prompt “Resume Editing Deck?”, the format change was successfully applied.

Same issue for me on Windows client.