no easy way to shift the spans. best way i know of is to do the following:
1. press PP to show all strokes.
2. (optional) press caps lock to disable rendering for a little bit.
3. select all, then shift strokes in time as needed. (try to make a note of which strokes were on the previous base frames, and the lengths of each span.)
4. for each span:
a. delete the span.
b. move to the new location for that span's base frame.
c. draw a Roto Brush stroke to force the creation of a new span.
d. delete the created Roto Brush stroke.
e. repeat a-d for other spans.
please file a feature request if you would want an easier way to do this.
thanks,
:jeff