Is it possible to....

Is it possible to get a upper reward seat outbound and a premium economy reward inbound, and then pay to upgrade to upper inbound. If so Is this likely to be an expensive fare class? Just trying to figure out if its worth us boosting our miles on an upcoming trip before the' extra booster miles' ends.

Nope, you can only upgrade a reward seat with miles, the only possible exception being at the airport where if availability remains they may allow you to upgrade with cash.

Could always buy the miles though?

Or look into booking a Combi fare

it may be cheaper to pay for E tickets (or PE) & upgrade them with miles. 12.5k miles to go E to PE or PE to UC & 25k miles to go E to PE from Man to Mco (other thread you said was looking for 3 people Man to Mco)

If its a case of you not having enough miles. You would need the correct fair bucket (so will be more expensive than the cheapest advertised) to be able to upgrade (so a call to V helpdesk would be in order)

I think in another thread you was looking to go UC
If you are only going to go one way in UC, then it may be worth considering coming back in UC as the taxes are a lot lot cheaper & you could put that savings towards upgrading your outbound flights!