The new Photos app is no longer iPhoto. It's a brand new app that contains most of the features of the older app, and distills some from Aperture, but not the power user capabilities.
In order to get Photos it has to be with Yosemite 10.10.3.
You shouldn't have a problem updating. The problem will only impact apps that aren't compatible with Yosemite, and I can't list those here (best to just check Apple's discussion forums).
What appears to be the case is that the latest Yosemite update fixes loads of issues that caused trouble for users who went to 10.10. If you want to be cautious, just wait a few days and see if there are reports of any serious problems. There will always be something, but my Yosemite experience, particularly with 10.10.3 and Photos, has been great.
I can't tell you whether iPhoto will ever be updated again. Perhaps for bug fixes.