You can install and use MC15 without altering your MC12 instance anyhow. The old version will continue to work as before.
It might be a good idea to first just explore the new features and options with a minimal library and import only a few test files.
MC15 uses some redesigned file tags that are incompatible with MC12. To preserve your old file tags you can disable Auto-Import (Options > Library & Folders > Auto-Import > untick: Run Auto-Import in background) and file tagging (Options > General > Update tags when file info changes).
When you are ready to try your full library, simply create a library backup file in MC12 (store it in an easily accessible location) and restore the file in MC15*. You can still keep Auto-Import and Tagging disabled to prevent MC15 from doing any file changes.
The restored library will be separate from your MC12 library. MC15 will overwrite an older library (i.e. update the database version) only if you set a new library in Library Manger to use directly a path that points to an old library.
If you later decide to uninstall MC12 and you have cover art files stored in the default location (under the MC12 application data location) you will need to move them before uninstalling MC12, but you don't need to worry about that yet. MC15 has a tool that can move the cover art files.
EDIT: * When restoring, don't tick the new option to restore also the settings. It works only with library backup files that are created with MC14 or newer.