we are trying to streamline our coding process and part of that is trying to change the constituent code for all donors to something specific such as "individual", "organization", "foundation" and so on. It is the case that most of our constituents have the code of "donor"...

My advice would be to break your import into small chunks rather than tryng to pull it all in at once. Start with the Bio information and nothing else. Then globally add whatever you can, such as attributes, constituency codes, solicit codes, add/sal, etc. Then bring in the address and relationships...