Renewing device certificate - safe?

Might be helpful to identify which device certificate you're referring to.

If you're referring to the device certificate for UI admin page, that would be here:

/config/httpd/conf/ssl.crt

http://support.f5.com/kb/en-us/solutions/public/7000/700/sol7754.html

As mentioned, this shouldn't affect connectivity through the SSL Virtual Servers.

The device trust certificate for HA would be here:

/config/ssl/ssl.crt/dtca.crt

http://support.f5.com/kb/en-us/solutions/public/13000/900/sol13946.html

It would be best to take a maintenance window if updating these since you have to break the device trust to update them.