The implication in the FAQs is that it is a speed boost a bit like TalkTalk used to have as an option. So you still have Value or Extra, with the unlimited overnight, and add the Fibre.
5. Who can get it?
If you're a new Plusnet Value or Extra customer
Once Plusnet Fibre is available (and if you can get it in your area) you'll be able to add it to your service when you sign up.
If you're already a Plusnet customer
Once Plusnet Fibre is available (and if you can get it in your area) you'll be able to upgrade to a fibre broadband connection through our Product Change tool.
7. What does it cost?
If you're a new customer
You can buy Plusnet Fibre from £16.49 a month.
If you're already a Plusnet Value or Extra customer
You can add Plusnet Fibre to your current package from just £10 extra a month. If you're on an older package you can change to either Plusnet Value or Extra and then add Plusnet Fibre (if it's available in your area).
That what John Hunt thinks too. Though he could be wrong I expect he is taking it from a Press Release like his quote.