Hi @Jonts,
Welcome to the Community! Happy to hear you’re creating a more personalized experience for your users by collecting their birthdays!
Yes, you are definitely able to Collect Date Properties from a Legacy Form. As long as the user inputs the same email address in the form when adding their birthday as is originally stored in Klaviyo, the form will update the profile property to the user and not create a new profile. However, this will require custom coding for this reason I would suggest giving the manage subscription page another look as it is the less technical route and Klaviyo doesn’t support troubleshooting custom code. If you do run into an issue when installing the form, I would contact your developer or reach out to a Klaviyo Partner.
Finally, I’d check out other conversations users in the Community have had about a similar topic that could give more insight!
Thanks for being a part of the Community!
-Taylor