They updated Kodos.It's unlikely they will go back to old mounts and update them. Not impossible but unlikely.
I feel the mounts current show a nice cycle of art progression. They're not easy on the eye but demonstrate how models and art have improved over the expansions.
This of course is not a reason to not update them. I just feel there's little reason to.
On a side note have they EVER updated old mount models? I can't seem to recall that ever being the case unless the model or particles were outright bugged in a way.