For example, if I own a subscription of Antivirus Pro or Internet Security will I be able to upgrade to Avast Premiere?
If so, would I pay only the difference in price?
If not, what are my options?
Would I have to just wait until my current subscription runs out and purchase a new version?
Thanks for all the help!
Hi Nick, welcome to the forum 
Personally I am using Avast Internet Security, and Avast is offering me an upgrade from the GUI to Avast Premier for the rest of the duration of my license.
I presume it will be the same for Avast Pro.
If the offered upgrade doesn’t suit your needs, post here on the forum, and we will put it forward to Avast.
Greetz, Red.
avast is currently working on a new (cross-platform) licensing system.
They expect prices to be lower compared to as they are now.
It will likely take a couple of months till it is ready.
Don’t know how long your subscription is still valid, but I would say wait till the new system is in place.