Thanks @Simsi and @MikeNoLastName
Simsi, the changes are not so extensive to create a new page for each existing page.
MikeNoLastName, my sites are always on the radar whatever we do or don't do. The competition on this particular site's topics is tough. Yes, changes are risky and it will be done with great cautious.
I made many self researches on my websites to learn whether Google takes page performance as a factor to decide ranking (p/v, BR, exit, ToS) , but never seen constant results.
Maybe they're still working on it, or they don't have reliable sources, I don't know.
At least, it's logical for them to put these factors at front when considering page quality.
The site contains many pages. These 300 are just a portion of its total and their metrics are the worse.
Now, the planned changes are meant to give readers the best experience, never to manipulate ranking.
There's no doubt that the pages are going to be improved, however in the past when improving pages (not these but others) results were mixed.
I share some of tedster's hesitations about changing a well-ranking page.
I do too, and that's why we'll never touch the top ranked ones before seeing how it works on the lowest ranked.
If ranking is not changed, I am fine and can live with that :-)
I would not repeatedly revise a page in any event, particularly not in reaction to ranking changes.
As far as I can recall, G has never changed their ranking. Must check again to be sure.
These particular pages are not optimized at all and wouldn't be.
I do think that constant fiddling with a page can make Google think you're trying to manipulate results.
The pages haven't been touched since they were published.
I hope Google is not so sensitive to trigger 'red alarms' for changes, but 'green lights' to recheck page performance.
The web should not be static. Things are changing and getting better.