Try to...
0. Close the Bitcoin client.
1. Install your FIRST FILE.
2. Remove database folder.
3. Remove .lock & .00* files.
4. Remove blkindex.dat file.
5. Remove blk0001.dat file.
6. Open the Bitcoin client.
7. You'll get confirmations.
8. Close the Bitcoin client.
9. Install your SECOND FILE.
10. Then do steps from 2 to 8.
This is slow, but should work.