If you convert an existing compressed audio file you have to a higher bitrate, no, there's no improvement in audio quality. If you put the CD back in your computer and rip at a higher bitrate, yes, there is an improvement in quality.
I'm pretty sure you can re-import at a higher bit rate without losing your play counts. Try it with a CD that has only a couple of plays. But make sure the album titles and song titles, etc. are exactly the same on the CD as in your library before clicking IMPORT.
If that info is the same, iTunes will ask you if you would like to replace the songs. Click yes and the play count and playlist info should remain intact.