Changing a herds discipline?

So I recently acquired a young foundation herd of western quarter horses. Since I already have a small herd of western quarter horses, I was thinking of switching them to my preferred quarter horse discipline of endurance. Should I do it now while they are young or wait and get a foal out of them for western before switching the to breed for endurance. 

I would see how prevalent endurance qhs are.  If you're thinking about selling a set of foals, and end qhs aren't popular, you can always breed them as western, sell the western foals, then transfer and keep the endurance ones for yourself.  If endurance seems popular enough, I've found hay cubes + riding school to work out to higher stats on average, so you can always cube them up, transfer their specialization, and then breed them.  

I'm currently doing the cube method on my set of paints, who are set for sj, but will be switched to western before I breed them.  The method seems to work the best with foundies, when they're in the lower show brackets that are a lot more crowded.