Question about updating OS and iPhoto

I got a call from my in-laws today about issues with their computer, their preferred browser not working, etc. Well, after looking at it all, the main issue seems to be that they’re still running El Capitan 10.11.6. As a result, apps are slowly losing support. However, they are terrified of losing iPhoto, which is apparently why they haven’t updated. My father in law spent hours and hours creating slideshows in iPhoto with special effects and set to music, very sentimental ones. My question is, will Photos preserve the slideshows from iPhoto? I have a bad feeling that Photos will not maintain what he’s created, which is very important to him. Thank you for any help or insight! And if the answer is “he’s going to have to remake them,” at least he can start preparing for that.