Sorry, I didn't mention the versions.
Before update I had wampserver 2.2c 32bit and updated to 2.2d. MySQL version in both is 5.5.20 judging by readme.txt file.
I tired to reproduce it in steps:
1. Shut down apache and mysql services
2. made a backup by zipping contents of wamp directory
3. installed 2.2d without uninstalling existing 2.2d version
I used WinMerge to compare mysql5.5.20/data/mysql directory with backed up and it appears that installer replaced users.MYD, users.MYI, db.MYI and db.MYD with default empty files. Other existing database directories are intact. Although update didn't replace my.ini which was modified.
I created a paste with WinMerge report in CSV as a proof - [
pastebin.com]