Moving our legacy hubspot forms to the new form editor, anything break?

Hidden fields. If you use them for UTMs, campaign or lead source, check every one made it across. Nothing looks broken on the page when they're missing, the data just quietly stops arriving and you find out at month end. Same for conditional fields and whatever happens after submit, the thank you message or redirect.

the stuff that bit us: - embed codes. treat every form on the site as needing to be re-embedded and checked, don't assume the old one just carries over. - custom CSS. anything you wrote against the legacy form markup can stop applying, so budget time for styling. - workflows, lists and reports that filter on "submitted form X". if the migrated form is a new form, those filters still point at the old one. go through them before you switch. do one low-traffic form first and submit a test lead all the way through to the CRM before touching the rest.