Will old apps/tools/plugins remain updated or else I would be required to reinstall all of them?
All apps that still exist in the new version will get updated, no need to reinstall
Will it erase user's old data files from disk?
All user data will be kept (although it is always a good idea to have a backup)
Will it remove my Internet browsing history, cookies, password?
Everything that resides in your home directory will be kept
What major changes are expected post upgrade, any precautions or back up to be done?
I am not aware of any major changes, although the UI is different. And (as stated above) it is always good to have a recent backup of important things - especially
- everything in your home directory (don't forget the hidden files/directories)
- everything in (and beneath)
/etc
, the directory for all system settings
Cited from Upgrade instructions for debian systems
Before upgrading your system, it is strongly recommended that you make a full backup, or at least back up any data or configuration information you can't afford to lose. The upgrade tools and process are quite reliable, but a hardware failure in the middle of an upgrade could result in a severely damaged system.