Apple Card Family Share - I selected participant but want to make a co-owner. Is there a way to convert or do we have to cancel the card and start over?

OK, you need to remove the Participant following the steps in this support article linked below,

Remove a participant from your shared Apple Card account - Apple Support

The card the participant receives will be canceled so it’ll be no good.

Then follow the steps in this support article to send an invitation to be a co-owner.

Set up Apple Card Family and add a co-owner and participants - Apple Support

Once approved as a co-owner a new titanium credit card may be ordered.

Request and activate a titanium Apple Card - Apple Support

I found the offer online,

https://offers.applecard.apple/apple-card-family-offer

The fine print is at the bottom of the page, linked above. My reading of the offer is it must be a new co-owner, they must be added to the account (they must qualify financially) by September 12, 2023 and within the first 30 days spend $100+ dollars to receive a $100 Daily Cash.

>>Apple Card Family lets you share the great features and benefits of Apple Card with anyone you call family. Add up to five users to your account by September 12 and they get extra Daily Cash. A new Co-Owner receives $100 Daily Cash when they spend $100+ in their first 30 days. New Participants receive $25 Daily Cash when they spend $25+ in their first 30 days. That’s up to $200 Daily Cash when you add a Co-Owner and four Participants.<<

How long ago was this? Have you ordered a card for the participant?

Does anyone know if following this procedure to change a participant to a co-owner will trigger the $100 bonus?

Do you have the terms and conditions of the offer? If you can post a screenshot or a link to the T&C’s we can tell you. Is the offer in you Apple Card in your Wallet app on your iPhone?