Certificate for ZDM expiring, iPads

Hi all, 

We have several iPads as controllers that are not supervised but are on ZDM, and our ZDM certificate is coming up for renewal.  Is it necessary to actually update the certificate for it to work?  Or does it just no longer receive commands from the web if it is expired?

Also, what is the best way to update it?  I can't recall what we did last year, but I seem to remember having to reinstall the Zoom Rooms app.

Thanks!