It's always a shame when a triple Gold Chevron unit has become defunct and needs to be disbanded in order to recruit higher-tier units. It feels like a unique part of your campaign - this unit that has followed your Lord to Hell and back - is just getting traded out for a new, ultimately generic unit. This weakens the attachment many of us feel with our extended campaign.
My suggestion is this: allows us to spend a sum of gold to deposit that XP 9 unit into an RoR-like pool. The unit is taken out of its army and given a 10-turn timer before it can be put out again. Maybe for 1000 gold, we can keep that outdated unit of Spearmen with Shields for a day when the Empire finds itself in dire need. After it is summoned to a new army, it is treated like a regular (XP 9) unit of its type; therefore, if it is ever fully wiped out in campaign, then the unit is gone. In order to deposit the unit back into the "pool" in the future, the money would have to be spent again, just as the first time.
It seems like this could be implemented within the framework of RoR, Raise Dead or "Blessed" systems, but apply to essentially any faction. I'm interested in thoughts on this concept. I think it supports flavorful campaigns, dynamic campaign strategizing, and is not a particularly novel or revolutionary direction mechanically-speaking.