Actually, if you are looking to switch, I would do so on a new game instance. I use TES4 Manager to manage muliple installations.

The reason I suggest that is because on an existing game instance, OBMM is in control of your installation. So you have to go through and uninstall all your omods. OBMM can remove the files that belong to an omod ok, but if there was a conflict with another mod, the files in conflict are removed to.

I guess I'm trying to say that rather than trying to remove all your omods and deal with the potential for loose files hanging around that need to be cleaned up manually, create a new game instance using TES4 Manager. Then you can use your old game as a list of all the mods you want to install. In the new game instance, you can slowly copy archives into the BAIN installations folder and install them. It allows you to learn BAIN without fulling committing and screwing up your game. You can still switch to the old instance and play if you wanted to.

The one thing I really like about BAIN is the ability to drag the packages around so that you can create an installation order. I used to have to do this with a separate list. So for example, I can install my UI mods first and then other mods that need to override parts of the UI.

So I wouldn't try converting an existing game. I think that would be pretty messy. But with something like TES4 Manager, it's pretty easy to create another game instance and you can try BAIN with that.

There is one benefit though to doing the switch on your live game instance. Uninstalling all the OMODs, since that should remove all the files they used, will reveal stuff that's dangling that may or may not need to stay. Ideally you should end up with a Data folder that matches a vanilla game install once all the OMODs are uninstalled. There should only be leftovers if manual installs have been done.

I think in this case mTES4 Manager would confuse things needlessly.

As you make a new BAIN package ... uninstall the OMOD first - then install the BAIN package. if you install the BAIN package then uninstall the OMOD the BAIN package may overwrite the OMOD but the OMOD will remove those files again. Not the end of the world considering that BAIN (unlike OBMM) will tell you that supposedly installed files are missing and with on click you can re-place them.

You can do a few replacements over to BAIN at a time and then you will see that the more you BAIN the more control you ... er ... gain (sorry bad poetry).

In the long better to have all with one installer and since only one installer truly addresses file over- and under-writes dynamically (on the fly) then that would be BAIN.