I discovered an easier way to manage version changes between a parent and child workflow that loop through and save multiple reports. Simple layout changes can be copied and pasted as mentioned above, but larger scale changes to report prep get trickier.
I incorporate the JSON conversion and return to table prep into my parent workflow to mimic / avoid difficult to trace errors in the child workflow. Once I have a newer version built / tested / saved in the main parent workflow, I just delete everything but the JSON return + prep and the report builder area, then do a save as to create the new child workflow, then paste in the loop call from the prior version.
Another trick is that I create a local table with the data for a single report run in the parent workflow. I keep that in the child workflow unconnected for troubleshooting and simple report layout changes.
I doubt many users go this far down the BIRT rabithole, but maybe this will help save a few people some hassle when managing looped through complicated report designs.