Here are a few ways to do this:
1) Turn on Settings > Account Sync > Personal Sync > Lyrics. Then the song edit pages will show a Shared Lyrics and a Personal Lyrics field. Enter full lyrics into Shared Lyrics and abbreviated lyrics into Personal Lyrics. For performance, turn on Personal Sync > Lyrics and for rehearsal, turn it off. Pro: easiest to set up. Possible con: only one user can see the abbreviated lyrics.
2) Add a custom field and turn on the Multi-Line option. Enter abbreviated lyrics into that custom field. Duplicate your layout, and add the custom field in place of the Document Viewer in the new layout. Use the original layout for rehearsal and the new layout for performance. Pro: easy to set up, can share the custom field and layout with other users, can show the abbreviated and full lyrics in the same layout if desired. Con: can't use per-song text sizes, auto-scrolling, remote control or line highlighting in a custom field.
3) Create documents containing the abbreviated lyrics and attach them to the songs in the second position, after the Lyrics. Change Settings > General Settings > Defaults > Document to 1 for rehearsal and 2 for performance. Pro: gives more control over the abbreviated lyrics layout if needed. Con: harder to set up because it requires creating documents in another app and loading them into BandHelper.