What's the easiest way to "lock" a page and disable all inputs based on a field value?

I basically have one page that has lots of input fields on it which is for the “Leads” data type. One of the fields is called “Stage”, so if that field is updated and saved as “Converted” I want all other input fields on the page to be disabled. Other than doing a condition against every single field, is there any way to do it just once for the entire page?

Have you tried putting a transparent group on top of all the fields that isn’t visible on page load and then is set to visible when the stage is reached?