if by pay as you go you mean that there is no charge for the service and you just pay for the time online through your phone line then there is no reason why you shouldnt keep the account open and setup outlook express (or whichever e-mail prog you use) to check both your new and your old account.
On NTL, during the change from dialup to broadband you can keep the same e-mail address but you have already said that isnt possible.
it seems you have three choices
1, upgrade with the same isp.
2, do as i suggested above if your dial up is a free one (no monthly charge)
3, ditch the old e-mail address, upgrade to BB and start afresh (not a good option)