Under MS Project if you want to make your updated plan as a new baseline, just save a new baseline using the menu command for this purpose depending on MS Project version and the data will be frozen on baseline fields. These fields will be kept across your updates and will only be changed until you re-baseline the set. MS project has a limit on how many baseline data sets can save, take note that it saves only a few selected fields.

I would recommend for every update/new baseline to keep a separate file, then from any prior update you can work new baselines as best fit, if need be. Then using export/import functionality within MS Project transfer baseline data only. In this way you can overcome the limitation when your software only allows for baseline data to be saved within the current project files without direct access to other project files. Because MS Project Baseline fields are limited, unless you keep separate files of the updates you might lose track of other relevant data, always keep a separate file for every update. Prior to any update save latest version to a new version file and then do your update on this new file.

Other software allows you to save and access your baseline data to a differently named file. This is easier and gives you more flexibility, you will have no need to do cumbersome data import/export.

In any case there is no need to double entry.