These are only two of my many imaginary friends. Speedy decides that the best way to provide backwards compatibility is to write a converter which simply converts 1.0 version files into 2.0 version files. She starts banging that out. Type, type, type. Clickety clickety clack.
Hard drives spin.
After about 2 weeks, she has a reasonable converter. But Speedy's customers are unhappy. Speedy's code will force them to upgrade everyone in the company at once to the new version. Speedy's biggest customer, Nanner Splits Unlimited, refuses to buy the new software. Nanner Splits needs to know that version 2.0 will still be able to work on version 1.0 files without converting them.