Seems easier to just go monthly in August on RHH and then do an add on track in Sept which will net the same price as doing a 3 month sub ($119)I am in the same boat as you. I am thinking about signing up for this month for a 3 month Rap/Hip-hop membership now and then reaching out immediately to convert this months ROTM to store credit. Then when in Sept when I would want two both the R&HH and Essentials track reaching out to CS again to see if I can have some of the store credit to cover the cost of the Essentials track to cover the cost of that.
I guess your way would net you about $6ish in credits left but that little amount doesn't seem worth the extra effort.