I've been tasked with creating a form that will only ask for email and country- UNLESS the lead is known (aka in our database). If known, the form should require additional info (name, company, etc.). Is this even possible?
Let's be specific about "the lead is known." Do you mean
If their session is anonymous, you cannot know until they've filled out the form that they would be merged with an existing record.
Hence you have no idea, at the time they first see the form, how many fields to display.
Forms can switch fields depending on the characteristics of an existing associated session (hiding the form if the session is associated, for example, or displaying only empty fields). This is what Progressive Profiling and other features are all about. But it can't work when the server hasn't seen the person yet. Does this make sense?
Read up in the docs and previous Community posts on Progressive Profiling.