It all depends how you broadcast your network outside of ACI domain.
3 Options come to mind:
1. You are using "GOLF" as L3 out . This allows you to send "/32" host routes from ACI to outside of network. With this option, the WAN will learn of the new location of the VM and route traffic to the correct POD.
2. You are using standard L3 Out and sending network routes via OSPF outside of ACI domain. In this instance, WAN traffic will reach POD A and if VM is on POD B packet will be routed through IPN to POD B until it reaches Leaf that holds new VM at POD B.
3. Starting from APIC version 4.x , you will be able to send "/32" host routes with standard L3 out .
* I personally prefer GOLF option since it allows me to use Spines as "Border Spines" and all L3 outside from ACI is going through SPINE to 7K.
* Also, GOLF allows you to easily create several L3 out VRF's automatically.