Celestial Charger as Paladin Mount


At lvl 5, a Paladin gains the ability to summon their Paladin mount. However, at levels 6-8, they gain the ability to summon a different, unusual mount. At lvl 6, a Paladin gains the ability to summon a Unicorn as their special mount.

My question is: Is there any way to eventually upgrade this Unicorn to be a Celestial Charger? Of course not (necesarily) directly at level 6.