e.g. FREESERVE and BT INTERNET let you keep the address, as whichever package you choose (except maybe broadband) automatically sign you up for a "NoTies" / "Pay-As-You-Go" for when their other services are busy - which you use another number to connect to. You may never have done this whilst with virgin - but if you had a pay-as-you-go account (i.e had to pay a few pence per minute spent online) then (this is true of many other ISPs, I don't know about virgin specifically) you will just need to connect to their pay as you go number once every months or so (check terms and conditions). I have just changed my address with freeserve (changed from HomeTime to AnyTime) and now have to do this to keep my old address going!
Hope this helps (that's if you can even make sense of any of it - I hardly can now re-reading it!)