Depending on the way your taxes are applied. Either keep it, if you have a high risk of getting taxed on the point of sale/swap or don't delay it.

If the purchase is old enough they may not have bank records. How long back is the legal requirements for holding transaction logs in your country etc.

I would preferably swap them into monero in batches and then transfer them to a new clean Monero wallet. After that you can swap them out again to an anon bitcoin wallet or keep them in monero. But, it depends on your situation, make a test before the year ends and see what happens.