Xeos is right about the 10exp limit.If you upgrade a unit, it will lose always all experience beyond 10 exp, but it will keep its promotions.
Implications of this:
- consider to promote only "green" units below 10 exp.Older veterans might be still useful and able to get high level promotion.If you upgrade them, the chance to pomote any further is reduced drastically, since you have to go the long way from 10 exp again.
- if you decide to upgrade an elite unit at all, make at least sure that the unit isn't about to be promoted soon.Upgrade it immediately after it has got a promotion (for example when at 17/17 and under no circumstances if the unit is at 16/17!), this way you will at least waste no points.