First off I recommend a couple of backups strategies:
- Make sure that you have at least one Time Machine backup of the current mail data
- Use a mail archiving app such as Mailsteward to make a separate copy of the emails in the form of a searchable database:
Smart email archiving app? - #21 by gastropod
My Mailsteward archives go back to the early 2000s. Once in a while I copy the archive to a Bluray disk (50GB) for long-term storage. Recently I searched that database and managed to restore some lost emails to an mbox folder, although Apple Mail refused to open it and I have to use another app (see the 29th post in the above link).
Regarding your question, I suspect there will be difficulty moving 25,000 emails from various email accounts to a fresh user account on your Macbook Air. The recent email accounts like iCloud should be okay to deal with as you just log into the email account as a new Mac user and the emails should copy over from the cloud.
However emails that are not cloud-based will, I understand, not be easy to copy. From my experience these mailboxes seem to be encryted and tied to the current Mac user account and so the mbox folders cannot be copied and opened with another Mac user account.
Others may have tips to do this.