There are two ways to move Tab digits to different lines while keeping the pitch the same:
1) If you want to shift several notes/chords at once, select the measures you want to shift (ie. by dragging the mouse over the notes, or hold Shift and hit the right arrow key) and then go to Actions | Set Tab Fret Position. When the dialog shows up, enter a new "base" fret position (minimum fretboard position where all notes should be played) then hit OK. The Tab notes should shift to the new fret position.
2) To shift one or two specific notes, right-click the mouse so that it is immediately to the left of the Tab digit you want to shift, then hold down the Ctrl+Shift+Alt keys simultaneously, then hit the up or down arrow key to shift the tab digit to a new line while keeping the tone the same. (Here's a mnemonic to remember this combination: you want to SHIFT the tab to an ALTernate string while CONTROLing the pitch to keep it the same).