This isn't too hard, but you will need to reposition the fields. I have done this before, but it is better than starting from scratch.
1) add the new stored proc to the report...Do not remove the old stored proc (just like adding a table to the report)
2) take the fields from the new sp and place them one at time on the report. I know that CR XI has the 'painter' function (right click on the old field, select format painter, click on the new field with the paint brush) which transfers font types/colors/suppression logic. Now click the new field , hold the CTRL key down and click the old field, make them the same size, then align left. at this point, I usually can delete the old field and use another field in the same section to align tops with.
Voila, the 'new' field has replaced the old field, in exactly the same place and style as the old field. Repeat until all the fields on the report have been replaced (I check by running the cursor over them and making sure that the datasource is different.)
Some of the fields may still be checked, Look at the groupings and the formulae. Altering a formula is easy, and the group is just right clicking on the section and selecting 'Change Group'.
When all the check marks are gone, go back to data expert and remove the old stored proc.
Yes it takes time, but it is way better than starting from scratch.
Hope this helps.