Think they'd prefer a BACS transfer, and yes, if you downgrade to PAYG you'd keep that (they put money in my account when I was in a house with no landline for a while), but then you'd not be a "new customer" so you would not be eligible for the types of discount on offer...
It was the idea that going to a new product, namely Fibre, on a brand new phone line (with rental paid to PN), but in the same house, was not eligible for the fiver a month deal, that annoyed me a bit, after several years with PN and adding the phone rental (because they have a value for money install price)
As I mentioned, 'academic' because the exchange wasn't sorted out in September (I've seen a new cabinet in a nearby street, however, so they are probably on course to sort it at the end of December - but there's not going to be many Openreach engineers happy to work in those weeks, with children off school and the Christmas / New Year celebrations, so doubt it will be possible to order anyway, until second week of January (!)
If you run a business, have a second ISP and backup web hosting...
1996? Enterprise (IoM 0345) + Clara.net
1998? Clara.net + Freeserve
2000 Freeserve 500kbps (+ Clara dial-up + Clara USENET/Mail)
2002 ? Eclipse 500 kbps + FS
2003 ? UKFSN 1000 kbps + Eclipse
2005 ? PlusNet 8 Mbps+ Eclipse
moved home - used office BB
2008 Three dongle at home
2010 PlusNet 2 Mbps + Three dongle
2012 PN 10 Mbps + Three (ZTE 802.11 router) + Three mobile (802.11 hotspot)
2013 PN 30? Mbps + VM 30+? Mbps + Three mobile