Cabin Upgrade on Reward Seat Purchase

Hi one and all,
So I have just recently booked reward seat sale flights in Premium Economy (LHR-LAX) and Economy (LAS-LGW) using the majority of my miles. If I wanted to upgrade the return leg to Premium, when I have gained enough miles/availability, what additional costs would there be? Is it just the difference in taxes?
Many thanks in advance!
So I have just recently booked reward seat sale flights in Premium Economy (LHR-LAX) and Economy (LAS-LGW) using the majority of my miles. If I wanted to upgrade the return leg to Premium, when I have gained enough miles/availability, what additional costs would there be? Is it just the difference in taxes?
Many thanks in advance!