I am planning to upgrade Ubuntu 15.10 to 16.04 and I was wondering if it will erase the installed software e.g., openCV, Unity Tweak Tool etc.
- Upgrading will replace your current software with newer versions of that software.
An upgrade can go wrong and if the end result is a reinstall your installed software will get removed.
In theory software could be removed if it is deemed obsolete. Or replaced by something new (open office got replaced by libre office at some point; though the impact of that was minor or close to nothing).
ALWAYS make a backup of your personal files. Not just when you do an upgrade. Every day. And once every so do a restore to test the backup. Prepare for failure.
I've been upgrading Ubuntu since 10.04 every 6 months and I've never lost any personal files or installed software.
But, after upgrading to Xenial, repositories working with Willy will become deactivated. So, you have to re-enable those repos (or PPAs) and edit the sources with the right information (distro name). You will also have to check if the developers are distributing their packages for Xenial (Launchpad for PPAs)
See snaps here: How to activate deactivated repository after upgrade?