At the moment, when you have new technologies, you can upgrade your existing troops to use new weapons, and new armour.
However, you can not change their group size, nor can you provide them with mounts.
I would very much like these two as options for upgrades for existing troops. It would mean that getting experience on early troops is a lot more valuable if they can be made into stronger troops later, while maintaining some of their experience.
I would suggest that a mount upgrade should cost some gold and the relevant number of horses or warg.
And a group size upgrade should cost possibly a level of experience from the existing troops, and some gold.
