Sounds possible because as far as I know the booking will carry almost no reflection of the fact it was made with a voucher, and therefore perhaps no flexibility differences.

avios.com has a very decent Live Chat function and the agents have always been able to answer my technical questions, so I suggest you get your answer from there.

p.s. you should have been able to book a higher class initially, anything except F (so it's only eco->biz shorthaul, but eco->WT+ or WT+->biz longhaul).