When adjusting a posted gift you should be able to keep the original appeal and campaign in addition to original fund. While you can change these on the split gift tab it is not actually recording the orginals so you are not accurately reflecting the change.

related suggestions:

http://rediscovery.uservoice.com/forums/137015-raiser-s-edge-discovery-topics/suggestions/3067029-ability-to-query-and-export-on-previous-campaign-a

http://rediscovery.uservoice.com/forums/137015-raiser-s-edge-discovery-topics/suggestions/3608323-offer-do-not-post-option-for-gift-adjustments