This is the reason I've been using ThinkPad T latptops for years - the removable drive slot (UltraBay). I have the accessory that allows a second hard drive to be installed in the bay, so upgrading is a simple matter of inserting the drive, running a clone operation, and then installing the cloned drive in the new bay. No fooling with external hardware, no fooling with the XP migration utility, no negative affect on the original drive.
I've done this about 7 times in the last five years, making the investment in the hard drive tray for the UlraBay very worthwhile.
One caveat: I'm not impressed with the current crop of clone tools. I carefully preserve an older copy of Norton Ghost, which works beautifully because Windows is not running when the cloning takes place.
I've yet to have a laptop hard drive fail, possibly because I only used IBM or Hitachi drives. My upgrades were for capacity or performance. I thus use my UltraBay HDD adapter and older drives for backups, which is very convenient and very fast.