1) The answer is, it might. First thing to do is backup all your project files. I would make copies of the actual folders and put them on another drive somewhere. If something happens in the update that you don't like, this makes it easy to revert. You can also look at the changes between each update here: https://support.smartbear.com/testcomplete/docs/general-info/version-history/index.html
2) No, you cannot go backwards with versions. If an old test is opened in 14 it will be converted automatically and now must be run in 14. If a new test is created in 14, it also must be run in 14.
What is your current setup? We may be able to help you plan your upgrade.