It is possible to transfer credit from one Vodafone Pay As You Talk
(PAYT) SIM to another.

Usually there will be a 14 day timescale for this and the number
you're transferring the credit from will be barred during this time
(so that the credit can't be used)

Both numbers need to be registered in your name, and if either isn't
you can call 17298 (25p charge) to do this.