Email replies reopening old cases - can't change ParentId on EmailMessage

Hello,

I have an org that uses email-to-case and we run into the issue where customers will search their email for their last communication with us and reply to it rather than sending a new email. This creates the problem of reopening a very old case but also associates the incoming email to an old case for a different support issue.

It seems like you cannot edit the ParentId of an EmailMessage record to associate the new email to a different case. Ideally, when someone replies to an old case, I'd like to be able to create a new case and relate the new email to that case rather than it being related to an old case and issue.

Does anyone know of a solution to be able to associate emails with a different case?

Thanks,

Tyler