miles flights using amex upgrade voucher

im i able to book 1 way ecom miles flight then book the return date
later and add in the free amex upgrades both ways?
the return dates i am looking for are not avaliable at present

The two times I've used them I've had to make the return booking at the same time which has meant I've had to be a bit flexible with the dates as the actual dates I wanted weren't available. The voucher can just be applied one way but you then lose the return part (ie you can't use the unused part of the voucher for another journey).Others may have a different experience.
DL