Having read through this topic I too rang Zen. I am on a market 1 exchange and yes I could still change over, unlimited download usage and approx £4 a month cheaper. No brainer as they say, all ordered. Thanks to the OP for putting this up!
It might not be Zen's intention to allow 20CN customers onto the unlimited package, as it would be somewhat nonsensical(*) to put up the Zen 8000 prices then offer, at least to existing customers, a cheaper unlimited product over 20CN. That said, if Zen accept your Unlimited Broadband order over the phone and implement it, good luck to you!
This has shades of the introduction of the unlimited FTTC packages, where early in the product life a phone call to Zen sales would often allow a move to the unlimited packages even if you were in minimum contract period on a capped FTTC product. Later on, Zen started holding people to the minimum contract term on the capped product before they were allowed to switch, which appears to have been their original intention.
Anyone on a 20CN only exchange who wants to cut their costs would be well advised to call Zen sales and attempt to switch to Unlimited Broadband as soon as possible. If Zen don't intend offering Unlimited Broadband over 20CN, they will likely realise in due course and make this product switch impossible.
(*) - 'somewhat nonsensical' as it might be one of those hideous inertia-based marketing strategies, where those who ask for a cheaper price get it, whereas the majority who do nothing land up paying extra.