Time machine doesn't remember the system it just restored from.

I switched MBP's recently, and used time machine to restore all my stuff. Though it hasn't backed up since I switched. I had to reassign my backup drive to time machine, now it wants to backup the whole thing again as opposed to just my changes since restoration. Do I have to delete all my old TM backups? How can I have time machine "see" the old backups it used to restore my HD?

Thanks.

New full backup after a total restore is normal and expected behaviour.. Might as well delete the old backups and start again.