Flyspray

This is possible with the mass operation feature of Flyspray.

Example for version, but works also for OS and other lists:

1. Tick all tasks in tasklist that have the old version in the tasklist (use also the advanced search for that to filter the wanted versions)
2. In the bottom massop form select the version in which you want move the selected tasks (do not change any other option in that massop-form) and submit the form.
3. Go to the project settings to the tab of the versions and check if the usage of that version is 0, so you can tick that version and delete that version. Done.

But: massop feature is currently deactivated because parent-child task-feature adds a lot of complexity that must be considered with mass operations (cross project task trees, task and project permissions, ..)

If you know what you are doing you can enable it in the code:
in include/modify.inc.php near
"case 'task.bulkupdate':"
replace
$massopsenabled=0;
by
$massopsenabled=1;