Hi Jason ,
The " VPN in use " error means that the VPN is used at a apolicy , you should remove it from the policy then do the chnage then put it back
To know which policy is using the VPN :
From GUI search for a policy having " lock" , The indicates that the action is tunnel
Then edit this policy , under action section : make it permit insead of permit & remove the mentioned VPN
After doing your needed change , put the action back to tunnel & select your VPN
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