Can unity be used in compatibility mode for older version of unity projects?

I am working on a project where I need to be using Unity 2017.1 and I already have other projects that I am working on in the newest version of unity.

How would I go about this?
If I go back to the older version, wont that screw up everything with the projects i have started in the new version?

Or is there a way to run Unity in a compatibility mode where I can make it work with outdated code? like windows can do with some programs that need to work with older versions of windows?

OR can i just have two versions of unity on my computer at the same time?

please advise,

thank you

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