Colcannon

I have a BA PP Amex, I just qualified by spending £10,000 for the companion voucher, though have not yet received the voucher.

I understand that I can downgrade the card to the fee free one and be reimbursed pro rata (i.e. ~50% of £150). IF I did that would it have repercussions to me using the voucher or reduce the validity to 1year instead of 2?
Now I've qualified for the voucher the 0.5 Avios per £ aren't so important as the fee it seems. Any comments?

Thanks