I used the contact us links on avast.com to ask the exact same question (does this replace APS or is Omni in addition to) and this is the response I got.
"Goodday! As we receive your request about the Avast Omni, And you are asking if we can add the existing subscription to that Avast Omni. And to tell you honestly we dont have any visibility to exchange into Avast Omni. if you want to do that you can call our hotline number to discuss your request this is out contact number 855-745-3255 or please do visit our website
www.avast.com or support.avast.com.
Best regards,
Jayvee Villanueva
Avast Customer Care"
I have no idea what this answer means... "We don't have any visibilty to exchange into Avast Omni."