I think this is possible, and within the rules.

If you've booked using a Barclays voucher, so paid the premium economy Avios price for a Business seat, you can change it to First. However what you can't do is pay the business class Avios price for the First seat, it will be priced as the First Avios price (and taxes). Worth noting the Avios price for First is sometimes very similar to the business Avios price (and sometimes even cheaper). So essentially you are making a change to the booking, where the Barclays voucher isn't being used for the outbound, but is still used on the return.

I've just priced up a similar situation - booked long haul in business using a Barclays voucher, and asked to change one of the flights to First. They priced it up and could make the change, although I decided not to proceed yet.