I sent them an email and they confirmed:
Once the new version is released, it will be a free upgrade for customers who purchased the current version.
I knew from their FAQ that you do get free updates for ebooks:
When you buy an eBook directly from us on our website, you’re entitled to free updates for the lifetime of that edition of the book (if we create a new edition, then that will be a separate purchase).
But I wasn't sure if this is a new edition or not. Maybe the cover picture changing means a new edition (plus reworking the whole book probably).