I've never tried but given the published 60 day refund policy of refunding an unused portion of a subscription and also that they talk or upgrading subscriptions as well as extending them, it looks as if an upgrade is possible.
The jPod team are really nice guys so I'm sure they'd upgrade a Basic sub for the difference in price to a Premium subscription. It's just good business I think. Can't hurt to ask them directly.
As for repeat looks at whats available in Premium.
I think it's a reasonable idea. They just have to protect themselves from abuse perhaps and make the login work for it. Maybe give a day pass to basic subscribers every few months to entice them into coming in. Actually Basic lends itself most to an abuse maybe. Give me a free pass and I'd grab all the pdf in a supermarket sweep.
But Premium really only works as an online thing. There's nothing left to download if you have Basic.
(I have a premium sub at the moment BTW, no supermarket sweeps!)