That’d be better if they had to upgrade Flash beyond a background pony (Three’s a Crowd) or a guard pony with only one line (Twilight’s Kingdom). But it also might provide a bit of confusion for the younger audience. As far as the young audience is concerned, they saw Twilight have a bit of a crush on pony Flash at the end of Equestria Girls. But to have him appear in say Season 5 or beyond and they’re just regular ol’ friends might look odd.

So really I think the preferable route is that Flash remains how he is was in Season 4. Just one of the few notable Royal Guards that cameos occasionally.