Is it possible to code this buff?

The previous age's upgrades are free once the next age is reached (for a particular tech).

E.g. man-at-arms upgrade is free in castle age and long swordsman upgrade is free in imperial age (but two handed swordsman and champion are not free)

This is just an example but it could be an interesting buff, especially if the civ is incentivised to build the unit early - and so they have to consider the cost of upgrading against the benefit of using the upgraded unit early.