The switch with the smaller VTP config revision will update the other one. In other words, if the new switch has a smaller VTP config revision than the configured one, it will receive all the VLANs you have created. If the new switch has a higher VTP config revision and no VLANs configured on it, it will update the configured switch and this will cause all the VLANs to be erased (empty VLAN config will overwrite the existing one).
See the following URL for more information on VTP:
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...