Time Machine backup error??

I recently had apple replace the hard drive in my imac. I restored my computer with time machine from an ext. HD. Everything worked perfectly. But now when time machine starts it gives me an error message "This backup is too large for the backup disk. The backup requires 813.61 GB but only 314.78 GB are available." I think it wants to do a full backup. It doesn't recognize the old backups. Is there anyway I can save the old backups or if I want to use the same ext drive do I have to erase the old backups?