Downgrade of c# dll through wix installers (.msi)

I have dll with  3.01.1574 and 3.01.1575 version i have msi to instal the same.

There is no problem while upgrading ie if I install 3.01.1575 it replaces 3.01.1574

But if want to downgrade from 3.01.1575 to 3.01.1574 it is not replacing dll.

What is the problem?

Thanks in advance