Was playing a game last night with the following settings Warlord/Tiny/Always war/OOC using Sitting Bull. Just playing a little defense. Went to upgrade my main city defender from rifleman to infantry and his XP reset. Went from 82/82 to 10/101. First suspected this when I upgraded that unit from a longbow to a rifleman. Wasn't watching careful enough to catch it then so i played til I could upgrade it to inf. Save is the turn that I upgraded the unit it shows its XP as 10/101. I checked the forum and the change list for 3.13 and saw no mention of this type of bug so I thought I'd throw this out there. If this is a "working as intended" design feature please ignore this post.

This is working as intended. All units with more than 10xp are reset to 10 when upgraded. Makes sense that when you upgrade a longbowman to a rifleman, his experience with a bow isn't going to count for much.

You do retain existing promotions though. it'll just take you much longer to get the next one.

Yep, it's working as intended. You can keep the unit's experience if you attach a Great General to it, such units are spared from the XP loss when upgrading.