Yes, it's possible to partially regenerate tracks. You can also freeze the tracks that have the material that you want to keep.
What I suggest is...
I know you've been around BIAB for a while so you probably know most of the below. I have included comprehensive information in this thread for anyone who reads it and is relatively new to BIAB.(1) Copy the track(s) that have the intro and bridge to utility tracks. Freeze the Utility track(s). This will also safe guard the intro and bridge.
The function to do this copying is "Edit | Copy Special | Copy/move tracks".
(2) You can then use the automated volume in Audio Edit on the copied track(s) to bring the intro and bridge in and out of the mix. You could also use the automated volume (audio edit), or bar settings (F5) to mute the intro and bridge bars in the original generation.
(3) Partial regeneration is possible in a couple of ways. The video below goes through those options.
The below videos will explain these things to you...
The below video is on BAR SETTINGS.
The following video is about FREEZING TRACKS
The below video is about AUTOMATED VOLUME (it is covered at timestamp 51:18)
The video that follows is about PARTIAL REGENERATION (it is covered at timestamp 6:45)