My Pro AV license is new, but I’m thinking of upgrading to IS.
So it there any way for me to do it by paying the difference instead of buying a brand new IS license when my Pro AV license still has about 300 days left?
Didn’t find a clue on the support site.
Not that I’m aware of. 
It’s OK not getting answer here.
But I’m a little frustrated cuz’ I submitted an email to support yesterday on official website and still hasn’t got any reply.
Yesterday was Sunday.
And one day?
Come on, you are not the only one contacting them.
Have patience.
Besides, if you look up you order details you can see for yourself if there is a upgrade option or not.
Hi, there is no direct update me to better version option but if you buy one more year license of IS then the whole license period will be updated to IS. E.g. you bought a PRO license which is still valid for 300 more days. Today you buy IS for one more year - be carefull to use the same ID, credentials etc. And you will get IS valid for 300 days + one year.
Feel free to contact our support team (https://support.avast.com/Tickets/Submit and Customer Service) with all your requests/comments.