ERPNext 13 vs 15

Hi guys, what’s up?
I wanted to better understand the improvements from version 13 to 15, I currently have a version 13 system, I wanted to know what are the differences and improvements of 15, and if it is already in use, because I installed it in a database that I have and it says it is version 15 (14.5) I didn’t understand very well.
And if I have some modules made for the company, will I have to change them, or will they run on the new system?
What if I end up having a lot of problems trying to update the old version to the new one?