Just letting you know that, it took 26 hours to update the sort index and doing the conversion of my 127108 messages (462 MB) on my (quite slow) 266 G3. Everything went smooth and I seem to have no problems so far. Naturally, I backed up the DB prior to converting to version 5.
PM 5.0 | OS X 10.3.3 | Powerbook G3/266 | 384 MB RAM | 20 GB HD

