You can convert tracks using the Advanced menu and selecting the format and bitrate in the Advanced pane of the Preferences. You can select multiple tracks for this to apply to. Depending on what you're converting from and to (format and bitrate), this can take a while. If you're converting form one compressed format such as MP3 to another compressed format such as AAC, then you'll certainly lose quality no matter what bitrates are involved. It's probably a better idea to re-rip the files from their CDs.