I want to make sure that I upload the files correctly and don't step on modified files from other mods.

I have Newsletter Subscribe already in use. This mod made numerous changes to customers.php which I have in my template directory.

I did a Winmerge between my customers.php and the How Did You Hear About Us customers.php.

I am confused on how to go about making sure that 'how did you...' doesn't take anything out of 'newsletter' and screw that up. If I have a mismatch on a line between how and newsletter -- which one wins? What is the mismatch is because how has original code that newsletter changed and letting how win would break newsletter. Do I have to winmerge each against the zencart original?

I hope I am making sense to someone. My previous mod work was just using simple file replacements. Now I can't just replace. I really have to merge the newsletter customers.php with the how customers.php. Right?

I am really nervous about breaking things as I add mods going forward.

This is for my www.rabbitinmyhat.com for now.

Please help.
Thanks
John