Clear LTM config - NOT factory re-set

Hopefully this should be simple - Is there a clean method to erase the LTM config.. i.e. leave all the base config intact self-IPs, hostnames etc but overwrite the bigip.conf content.

I tried loading a new bigip.conf file form tmsh but the device lost all config I had to re-configure the device from scratch, even though I didn't touch bigip_base.conf.

Thanks