Changing "Version" to None keeps the old version information.
If an issue has a version assigned to it and then I change the version to 'nothing' (as shown in Screen1.png) the properties (bottom of Screen1.png) say that it has no version, but the version is still listed as its old version ('2010-Q4 Config' in Screen1.png).
The old version also shows up in the version column within the list of all issues (Screen2.png).
If I sort to show all of the issues with a version of '2010-Q4 Config', these mis-labeled issues are not returned (this is proper behavior). With sorting, the end of the repository URL has "issues?version=2010-Q4 Config" and only returns the correct results.