Hi,
If a retailer is selling a brand new B550 board, it should have the latest BIOS already flashed or at least a supported one, but this is not a 100% sure thing as it could be a poor box that sat on a shelf for years.
I would also buy from a place where the return policy is good enough to send it back if it doesn't work.
= OR =
Buy locally, ask the sales person and tell him about your situation, maybe he can help you out.
Some small stores will absolutely help you.
=OR=
Buy a board with dedicated flash button and BIOS USB port so You can do it even without entering the BIOS.
Good Luck
The Englishman