Storage Space on Upgrades topic

Hi everyone

This may seem like a stupid question, but when new OS upgrades come out, do they replace the old one entirely or does it sit alongside the old OS? Additionally when apps update do they replace the old app files or does it mount on top of the already existing app? The reason I'm asking this is because my old HTC Wildfire S (which I had to root soon after using Link2SD to improve storage) had poor internal storage by default and the only for me to get an app to update was to uninstall it and install the latest update straight from the Play Store.

My current Motorola Moto G 4G only has an advertised 8GB internal storage and I can no longer update any more apps because it is full.

I've moved several apps already to my SD card (the ones I rarely use) leaving the apps I use daily on the internal storage for better performance.

Relating to SD cards, if I have apps installed on my SD card and I backup that SD card on NTFS external HDD for example, wipe my SD card and transfer the backup to the SD card, will the apps installed continue to work?

Many thanks

Will

Sent from my XT1039 using XDA Free mobile app