Unlike most of the posters here, I'm not an avast "FAN", just a user for the past three years.
I too have had this problem and done everything mentioned above, as in 'FULL uninstall' etc., then fresh install from scratch to get back to the free version. It worked.......
....for ten days, then just like the rest of those above, back came the 'upgraded to pro/trial/expired' message and all that junk.
BUT, being just a user, not a 'fan' I'm not going to fiddle-faddle about every few days or whenever avast feels like making me a pro, I'm just going to go back to AVG which I used for years till it got too slow.
But the point of this post and what worries me most, is that avast themselves MUST be aware of the problem. So
either they won't admit to there being a problem, which does not inspire confidence,
or it's a deliberate ploy to fool some free users into going pro, which is totally unprofessional. Meaning that I've lost trust with the company as a whole and won't be trying to fix it - and having lost confidence I'm also going to remove it from my Android because I've lost trust altogether.
Nice one avast - I'll get my coat and close the door as I leave. What a shame: it was so good until a few weeks ago.....
EDIT: I've just remembered that the first time it happened was immediately after I downloaded a program via CNET: despite my using 'custom install and refusing all the toolbars, extra programs etc., I got infected with surplus programs, avast did the above AND malwarebytes got disabled. Took over two hours to restore all to normal: The program was 'miditzer', available only from CNET and the developers are trusted and do not add malware or additional stuff to their own program.
I thought it was just coincidence (and still do.....) but if anyone else has had the 'pro' message after using CNET it may just be an answer and something avast can investigate......