Forms API - when to use + using contact reconversion
Assuming that is the case, my second question is... With the automatic form collection, contact reconversion happens automagically when the users uses a different email address to the one they'd used previously. Using the Forms API is there a way to handle contact reconversion in the same way?
Automatic reconversion works by merging property and analytics information for visitors based on cookies (more specifically, their HUTK value) included with the conversion data.
If you want to merge visitors with the same cookie while using the Forms API, you'll want to ensure that the hutk parameter is included in the call's request body. This allows HubSpot to merge visitor profiles and recognize returning contacts using the same mechanism that's enabled by default for automatic form collection. The HUTK parameter should be placed in the Context section of the request body; you can view a sample request here.