Upgrading to different cabin in return flight with Barclays voucher

[ffflight]

Hi everyone, first time trying to use Barclays upgrade voucher and (hopefully) first time flying business.

I’ve got enough points to make a return journey in business class (or whatever BA calls it) using the voucher.
However, it would be best if I could upgrade from Eco->Premium Eco one way and PreEco->Business the other way
for 2 reasons:
1- Not fussed about business class both ways, I’d rather spend less Avios
2- I’ll have more options on available dates

It doesn’t seem like this is something you can do on BA’s website. Is there a chance they can do this over the phone? Is it even worth trying?
Is there another trick I’m missing?

[exploring_george]

Yes you can do this over the phone. I’ve done this myself.