When users leave the organization, we recommend that they be removed from JIRA groups as a means of revoking their access. If the JIRA admin then wants to bulk-move a removed user's issues, JIRA fails with a cryptic error:

This field cannot be updated, its value will be set to the default for the field in the target environment.

We should either print a more useful error message, or modify the code to allow this operation.

See the thread at:

http://forums.atlassian.com/thread.jspa?forumID=46&threadID=11054