It looks like you have a little bit of time to plan this out.
If you are using the Windows client VPN, change all the users to connecting to a DNS name, such as vpn.company.com. Then one day you can update this to point to any other site or IP address you want and (assuming the MXs are using the same settings) users will continue to connect and not know any differences.
Another simple option is to simply configure an additional client VPN connection on their machines, and then ask them to simple start using the new connection.
If you are using AnyConnect, you can create "site a" and "site b" in the drop-down list of connection options, and then ask users to simply start using the new site connection.