Seems like I am lucky! The next day after giving my calculator back to school, my friend and classmate gives me a TI-83+ for free !

I just noticed it's OS is 1.14 and I thought that it may have some special worth for collecting. I have found a topic where people say upgrading is better, as 1.14 is unstable, but maybe something changed since then?
(I mean many people upgraded their OS and maybe 1.14 got rare...)

So the question is: Should I upgrade it to 1.19 or not?μ

Yes, I would recommend upgrading it to 1.19. This adds some things that may be needed by some programs/applications (mainly related to tokens) and may fix some bugs. And I'm guessing that someone has an archive of old OSes anyways, so there shouldn't be any special value in keeping an old OS installed.

Try to beat Illusiat 6 then upgrade. Illusiat 6 final dungeon has display issues in 1.15 or higher, but upgrading to 1.19 would be better.

Okay, thank you guys!
I won't have time to do that today but I will tomorrow, and I will post here if I have any issues while upgrading.