The old chips were more likely to migrate, I've read that Home Again, has changed the design to eliminate this problem.
Quote:
| HomeAgain is the only dog & cat microchipping product on the market today that has the Bio-Bond patented anti-migration feature to help ensure that the microchip will stay in place so that it may be easily located and scanned. Dog and Cat ID Microchipping | HomeAgain Pet Microchip |