Hi
On a tier-1 gateway, the SR is present only if you select an NSX Edge cluster when creating the tier-1 gateway. Kindly ensure all stateful service like NAT, Load Balancer, Firewalling is removed from T1 Gateway once this is done.
Removing the edge cluster from the T1 Gateway and select an Edges Pool Allocation Size as Routing. This ensure T1 as DR only gateway.
Impact to VM depends upon Associated stateful services to it. if it doesn't have any they shouldn't be any impact during this DR only Change.
HTH
Sandy