The new update framework is more restrictive in what it will admit as a valid field name. Many invalid field names were not caught by the old framework. We still would, however, like to allow users to repair bad field names by using the $unset and $rename operators. To do so, we will need to weaken the field name restrictions enforced for $unset entirely, and relax the 'source' field name restrictions for $rename.

Marking as QA required since we should validate that we can repair data with invalid field names via $unset and $rename.