2LIS_11_VAITM enhancement question

I need to add two new non-standard partner types to the 2LIS_11_VAITM extractor. Right now the standard partner types of ship-to, sold-to, and payer are being extracted. However I need to add two more non-standard ones.

Here's what I've done so far:

1) I've added the two new fields to the MC11VA0ITM extract structure

2) Added code to the user exit EXIT_SAPLRSAP_001 to populate those two fields on the extract structure.

The problem with this approach that I've taken is that the partner assignment at the time of the extract is populated on both the before and after images.

What I really want is for the correct old value to be placed on the before image and the correct new value placed on the after image which happened at the time of the change, not the new values placed on both images at the time of the extract.

Any suggestions for a different approach are appreciated. Please be as specific as possible in your suggestions. This is kind of new territory for me.