I am trying to copy files from one document library to another, and
while at it, convert choice fields on the source to lookup fields on
the destination. The lookup table already exists, and I think I have
the right code to convert to lookup values, but I can't get them to go
properly in the destination. I either get an invalid lookup field
error, or no values show up in the destination.

I basically find the display name of the choice in the lookup table,
get its id, and try to put that in the destination lookup field (using
a SPLookupFieldValue object).

Has anyone has to do this? (I may have to resort to copying them
first as a choice, then convert them within the same document library).