Upgrading in stages

Hi,
I recently booked LHR-LAX and JFK-LHR for next July. I used all of my miles upgrading the outbound leg to UC (SO excited! and thank you everybody who gave me tips before I booked!) but I am steadily building them back up with Tesco vouchers. At the moment I have enough Tesco vouchers to give me approx 13,000 miles while the conversion bonus is on, and to upgrade Economy to UC for the return leg it would be 20,000.
My question is theoretical at the moment as there's no upgrade availabilty to either PE or UC on my flight, but I wanted to ask now so that I have the information if anything opens up!
Is it possible for me to use 10,000 miles to upgrade economy to PE (ie grab it when/if one appears!) and then if I manage to get the other 10,000 and there's a reward seat available, upgrade the rest of the way to UC? Or would my upgrade have to be done all in one go, Economy to UC?
I recently booked LHR-LAX and JFK-LHR for next July. I used all of my miles upgrading the outbound leg to UC (SO excited! and thank you everybody who gave me tips before I booked!) but I am steadily building them back up with Tesco vouchers. At the moment I have enough Tesco vouchers to give me approx 13,000 miles while the conversion bonus is on, and to upgrade Economy to UC for the return leg it would be 20,000.
My question is theoretical at the moment as there's no upgrade availabilty to either PE or UC on my flight, but I wanted to ask now so that I have the information if anything opens up!
Is it possible for me to use 10,000 miles to upgrade economy to PE (ie grab it when/if one appears!) and then if I manage to get the other 10,000 and there's a reward seat available, upgrade the rest of the way to UC? Or would my upgrade have to be done all in one go, Economy to UC?