Probably should have clarified that in your first post then.
No one is going to know the situation better than you, so only you can figure out what to do. Short version is you can't have two instances online at the same time which you seem to understand. So if you can't turn off the existing install, then there are only a couple options.
If you want to try and keep the FQDN, this *should* work. I say *should* because I haven't tried it and you haven't mentioned what version of 3CX is on the old box. But assuming a relatively recent version of 3CX you can try this:
- Make a backup of the existing system and copy it off the server (with license key and FQDN)
- Generate a new Try key in the partner portal.
- Spin up a new instance somewhere with the Try key.
- Make backup of the Try key instance.
- Copy the backup of the try key instance into the backup location of the old server if it's something like Google Drive/FTP. If it's local you won't be able to do that so you'll have to change the backup location via the management console to something you do control.
- Restore the try key backup on the Windows box. This should blow away that instance and have the try key environment running.
- Now you can restore the old backup on the new cloud server.
This is very convoluted, not tested, and requires some skill. If you are ok with possibly losing the FQDN then the easier way would be to:
- Make backup of the existing system and copy it off the server. (without license key and fqdn)
- Generate a new try key in the partner portal
- Replace the key in the windows instance with the try key. This will free the old license key but the old FQDN will be tied to the try key (may not work as I know there are some restrictions with replacing keys and try keys)
- Go into the customer portal for the try key and release the fqdn.
- Restore backup to new cloud server and try to use the old FQDN.
There's no guarantee either of these steps will work, and releasing the FQDN does not mean it will be available to use again. Realistically you may just need to buy a new key, take a backup of the system without the key/fqdn, and restore with a new fqdn. That's about the only guaranteed recovery method.