Hi,
I can confirm what Amit said. You can easily upgrade from v6 to v7. And you can connect v7 to ACS (i.e. for now you can reuse some of your v7 data to create emails with ACS, by synchronizing profiles, audiences, etc., using the ACS Connect feature in v7). But there is no migration mechanism from v7 to ACS.
You can either use v7 and ACS jointly via the ACS Connect feature if you have complex use cases that can be tackled by v7 but not entirely by ACS, or you can get a new implementation of ACS if ACS can serve all your use cases.
Let me know if you have more questions or if this is not clear, as it can quickly become confusing,
Florent