I would imagine it wont change. There's no real reason for it to change; your IP address is mapped to a Radius profile, which contains (amongst other things) your PPP login name and password. All you're doing is changing how your internet connection is provided (FTTC rather than ADSL). So you should be fine.
Well, that's what I thought, but having placed the order I've received an email confirmation and invoice that amongst other things contains a new Zen username (zen123456@zen format), which implies a new PPP account etc. Shame really, I wasn't too bothered about a change of static IP, but it looks like I'll be losing my zen1234@zen format username (i.e. one of the older accounts) which I rather liked. Not that it makes any real difference I know, but for those that have been with Zen for over ten years, there is a certain something about having one of the shorter Zen account names....shows you've been with Zen a long time!