Am I correct on using 1:1 NAt (Whole Networks) in this scenario & how?

At my company we have multiple site to site VPN tunnels to our Sophos UTM9. We have a few interesting scenarios where I would like to know how to configure my solution.

1. We have a few new client sites with subnets in their networks that have already been used in our infrastructure. For example 10.18.1.0 /24 is used at one of our sites and new client is using this subnet is well. Can I establish a VPN connection to their network and then NAT that entire network from 10.18.1.0 /24 to say 10.19.1.0 /24??? 

Is it simply creating the NAT rule and it will work? What else would need to change?

Rule Type: 1:1 Nat (Whole Networks)

For Traffic From: 10.18.1.0 /24

Using Service : Any

Going To: My Network

1:1 Nat Mode : source

Map to: 10.19.1.0 /24

Thanks