If it's not broke, don't try and fix it, lol. Until the pony starts have trouble chewing and looses weight, then I'd just stick with what's been working

.
If he's toting kids around and is plump, then he certainly doesn't need more energy...
Her vet would be the best person to ask though, as he/she can look at the pony's teeth and give a better estimation on how he's doing with the current diet and when/if he should be switched to senior feed.