If the old phone is in good condition, CyanogenMod supports popular Samsung models and costs nothing - just a bit of time
It is running CM12.1 at the moment - and it's veeeeeery slow on that S3. All previous CM's also had that odd camera bug where it cant connect to the camera. The only "fix" that made it functional was the workaround that restarts some services when you press the camera button - which means that you need to wait for 5 seconds until you can use it.
It's just a pain to use. Which is why I replaced it now with a Lumia 640 that "
just works". Daughter is happy - so I am happy as well.
I had several Android and iOS phones (work and private) during the last 8 years and I was not happy with any of them.
Android is just a silly mess where no company cares about updating the devices
(also applies to the custom roms because people stop to support devices, or just never support some devices - or never fully support all of their features.), and iOS is an overpriced "High Security Prison" where the user is not allowed to do anything - all hail iTunes the sync app from hell. :-/
I still have to use an iPhone 5s at work and I despise that thing - but I might have my boss at a point where he will allow us to use Windows Phones later this year as I began to show him the benefits - he began to see the light when I showed him my Surface 3 Pro last year and now we replaced all of those dreaded iPads that no one here liked to work with.
Ever since I bought my first Windows Phone (Lumia 820) I can't see myself go back to android or iOS again. Yes the lack of apps sucks, but the platform provides me with what I want and expect from a daily driver smartphone.
What worries me is that Microsoft appears to loose the interest. They push back the W10mobile release date, they fire even more people now from their mobile division, and they don't fix blocker issues like the audio on the Lumia 930
