On Windows, the installer has a checkbox "secondary install".
You can then install into a new directory, like c:\lazarus3_6 or c:\lazarus4rc
It will then ask for a "config folder", where you select an empty folder, e.g. c:\lazarus3_6\config
or c:\config_for_lazarus3_6
This install will then be independent.
Before opening a project, make a backup, as the IDE may change it.
Or make a copy, using "publish project" in your Lazarus 2.0.6 install.
I don't know wich version of fpc came with Laz 2.0.6
So you may need to look at
https://wiki.freepascal.org/User_Changes_3.2.2to see if changes are needed, and what they might be.
Similar for differences in Lazarus (look at sections about compatibility)
https://wiki.freepascal.org/Lazarus_3.0_release_notes