Fields left blank in landing pages overwriting data we already have

I'm having a couple of problems when creating subscription landing pages.

Checkboxes

I have a checkbox for people to opt-in, which when checked it saves as 'yes' in the boolean field in our database. But if they leave it unchecked it saves it as 'no' - overwriting what is already there. I don't want the box being unchecked to act as an opt-out. Is there a way for unchecked boxes not to overwrite what's there? 

First Name/Last Name fields

The same thing is happening with these fields. If these fields are left blank, they will overwrite what is already in the database - even if there was previously a name in those fields. I only want them to update the database if those fields are filled in and not left blank.

We have a lot of landing pages/widgets, which all share certain fields and I don't want us to lose data if they don't fill in a field on one of them.

Thanks

Michael