So you would need to re install VMware ESXi on your boot os. Once the trial is up you can only activate it with a license key not a trial key again. I assume you have a second drive for your VM, so what you want to do is leave that drive alone, I typically remove it just in case I select the wrong drive awhile reinstalling the os.

You will need to reload the esxi host, if you have your storage on a different drive or appliance than it should be easy