I do a "Stretching out" at the beginning of spring training, where I can add to a pitchers stamina, but only at the cost of durability.
That way I can have plenty of SP prospects stretched out into viable inning eaters, while at the same time there are others who will never hack it as a SP and get moved to RP roles.
Right now I do 1 for 1, but I don't think it's totally balanced. I think next year, I'll do the first 9 one for one, then on the tenth it'll cost two DUR. That would limit me to only stretching a SP out so much per year, while still accelerating special cases.
This also provides me a path to converting RPs to SPs.