From 11:59 pm on October 15, retailers have no obligation to accept the old £1 coins.
Any unspent £1 coins after October 16, can be banked or taken to a Post Office. Donate them is also a good idea. But please be aware that the exchange arrangement is a temporary option so better to exchange the coins now to avoid not being able to in future.
What if you are given back an old £1 coin?
If you’re still being handed back the old £1 coin, you have the right to ask the cashier to give you the new £1 coin, if they have any instead, but retailers don’t have to comply with your request.