If you keep the RealTracks folder and Drumtracks folder as they were, the old RTs will stay in place.

A different method: I usually install a new version in a new folder (in my case E:\BIAB 2018) besides the old BIAB 2015 installation. Then MOVE the the RealTracks folder and Drumtracks folder (move because of their size) in to the new place, and COPY the STY files ST2 and XT2 files, WITHOUT replacing in case they're updated. But i makes sense to copy all the new Realtracks and Drummtracks content after all this, as many are updated over they years.
It always takes time to copy-verify and check the RTs though.

Also a copy tool like FastCopy (my favorite) or Teracopy could come in handy at times. F