I have make a couple of tests and it seems this feature works perfectly. Here are results of my investigation.
First of all, I downloaded 'Aqua' skin from Plesk CP. After that un-packed it on the server and corrected couple of fields:
<VERSION>1.0.1</VERSION> - changed
<DATE>2007-03-22</DATE> - changed
After that packed this info .tgz archive and tried to upload but failed with the same error. As I figured out further I just forgot to use '-z' option during packaging. So correct command was:
# tar -czf Aqua1.tgz *
Here I got package Aqua1.tgz. When I tried to upload it I got next message:
Error: The skin 'Aqua' already exists
So I had to change also following fields in info.xml file:
<NAME>
<LOCALESTRING language="en">Aqua1</LOCALESTRING>
</NAME>
<DESCRIPTION>
<LOCALESTRING language="en">Aqua1</LOCALESTRING>
</DESCRIPTION>
<KEYWORDS>
<LOCALESTRING language="en">Aqua1</LOCALESTRING>
Then I re-packed the package and was upload to upload it successfully.
Checked me changes and can see that all information have been saved correctly.
Hope this information helps.