Protect changes made in wp sign-up.php

Hi.

I have made lot of changes to my sign-up.php.

I read this article:-
https://www.chriswiegman.com/2010/02/using-a-custom-wp-signup-page-with-wordpress-mu/

But got lots of white screen!!!!

Chris does not do support enquiries!

Any suggestions, how I could prevent losing my changes at update time?

Thanks

johnv