Seeing as though it's ridiculous to actually speak to a representative about this, I'll put it up on here with the hopes someone may be able to help.
I currently have a full subscription to all Creative Cloud Products paying $76.99/mo.
I've just been emailed saying its not 24% off, so how do I change to this plan??? Or do I now need to cancel my current plan and purchase this one (which seems ridiculous)?
Promos are typically for new users. You are not a new user, so it doesn't apply to you. You could cancel your plan and register again, and hope Adobe don't pick this up and refuse you the offer, but be sure you are aware of the cancellation fee.
As rightly mentioned by Test_Screen_Name the promo offers are for new subscribers, however, the best option would be to contact our support team on chat to get more details, to start a chat click on the chat icon on the bottom right corner of this page: https://helpx.adobe.com/contact.html
Let us know if you have any more queries.