Sales Order Form Personalization

Hi All,

I have requirement with respect to Forms. This is my First time working on Form  Changes.Could you please guide me in detail how I can achieve below

Based on a Value in a table I want to Validate the "Invoice Delivery Method" on the Sales Order Form. Additional Header Information is DFF.

1) changing from either Hard Copy or Email to Email & EDI or EDI will be possible only if the EDI Status field on the custom form is “Active”.

A validation will check this and notify “Account EDI configuration is not active yet”

2)changing back from either