Experience Points deducted when upgrading?

[Fistleaf]

Occasionally I observed that some XPs got deducted when upgrading a unit. (Other times, I didn't specifically check.)

How many points are deducted and are Warlord units' free upgrade affected similarly too?

[clarkcd]

Any amount of xp over 10 is removed. The promotions and the levels remain. Units with a warlord attached don't lose any xp.