I'm booking PHX>LHR>DXB>LHR>PHX with miles this week - hopefully all in CW. I'll have surplus miles, so I'm wondering if its possible to upgrade just 1 leg to FIRST? Further, I'll still be collecting miles until travel this fall, so as I accumulate more miles, can I upgrade more legs assuming there is availability? I need to book soon because CW fills fast (surprised there's availability, actually).

Sorry if this has been answered before, but I did search for about 30 minutes. I'm buring up my stagnant miles from the days of using the rocket for TA travel and this will be an awesome way to turn 30!