I recently played on a private vanilla SMP map (Moonlight) and it has since become available for public download. Between the last time access to the map was available and now, I changed my username. I checked my username profile in NBT Edit (I snagged which id was mine from a SSP map's Playerdata folder) and confirmed that the downloaded map DOES [appear to] have my appropriate data (I checked the field "EnderItems" both in the downloaded map before loading and after (before: 13 entries, after: 0 entries)).

I actually had a similar issue when usernames went from being stored as the actual username to a UUID, but I found a work-around I could use. I can't remember the specific version, but I loaded the map in the latest version (with UUID) and my inventory, etc. was wiped. I re-loaded the download and launched in the same version we had played on and my inventory was there. I then upgraded and was able to keep my inventory. This fix obviously won't work for username changes because you can't just change your username on a whim.