Well, you could create a dummy song and test this feature to be sure (meaning if you are really worried, test it safely) but as long as you don't 'save' it you'll still have the original.
Use Save As instead.
Or make a copy of it first (before any of the above) and rename it to SONG-Safe.mgu or whatever.

.. but to answer your question; no. In my experience the track data remains.

The main changes I do notice are:

Track Labels change color (yay! That blue reminds me of '98 internet)

The tracks no longer automatically generate (all of them) unless I select them all and tell it to do so.

I can treat them like regular DAW tracks. This is important to me because once in RB I start 'assembling' tracks. I generate/regenerate sections, keeping what I like and cut/copy/paste to assemble the song. Sometimes one track at a time, sometimes multiple tracks..

I can now start using multiriff to test multiple generations at once for a given track (love this feature).

But you do lose the ability to click the Generate button and have all your previous work on the first 8 or 9 tracks written over ..
In BB I expect this behavior.
In RB I do not want this feature at all. Not sure if I'm in the minority on this or not.
To me it should open the BB tracks as expected, but just make them regular tracks to begin with. We're in a different program now.
I get that!

Make your sound your own!
.. I do not work here, but the benefits are still awesome