Upgrading veteran units: Increased upgrade costs instead of lost experience points

[lockstep]

Originally posted by Proteus_MST
The only thing I would change about the existing way XP and upgrading is handled would be, not to let units loose all XP above of 10.
Instead it would just be 50-80% of the XP over 10, so that the unit still retains 20-50% of the XP ist gained after the 10th XP (and therefore has to regain much less XP than currently to get the next promotion.

Although I like my proposal (additional upgrade costs) better, retaining part of xp would be better than the current status.

EDIT: It may be also easier to balance for single-player games, because the human and the AI's would play by the same rule (while the AI generally seems to get massive gold discounts when upgrading units).