Can someone tell me are JetStream and JetStart accounts portable?

i.e. can Dave from Christchurch (who has JetStart) on holiday up in Auckland
staying with his mate Mark who also has JetStart use his own account by
changing the login details on Mark's router so that all downloads get put
against Dave's account and not Mark's?

The above example is JetStart to JetStart, what about JetStream to JetStart
and JetStart to JetStream, and of course JetStream to JetStream?

As I understand it the speed is limited by Telecom at the exchange so using
JetStream account on a JetStart connection will only give JetStart speed?

Also what about using an Xtra login on a connection that is normally
connected through say IHUG?

Cheers, Jason

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