I would suggest in the general settings having the option to edit the emails to users, so they are not effected on upgrade, for example I would prefer to have the following sent to a user that is due to expire as I do not know where to edit this anyway.

Heading:
Your ad's expire soon <site title>

Body:
Hello there,

Thanks for posting your ad on <Site title> - we hope you've had a great response. Unfortunately your ad will expire soon.

(nice if ad picture and title can show here if not a link to ad)

Check the status of your ad at any time or re-post your ad. Log in to <site title> to find out more.

If you don't need this ad anymore, please delete this ad.
(underline is a link to login page)
Thanks,

The <site title> Team

Hi,

Thanks for your suggestion. We will pass this to our product management team.

Thanks.