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I contacted them asking more info about the upgrade and had a prompt and detailed response by email. I agree with smaimon: give it another try by sending a email giving all the details - I am sure that if it's just a matter of credit which was lost they will eventually reimburse you, although it might take some time. The UM+ may still be a bit "experimental" at this stage (eg. absence of voicemail - tarifs on the new map are not correct for US and Canada) but my sim seems to work very well, there are new very good functions (eg. balance received by sms).

By the way, I have transferred only part of the credit to UM+, so I still have my +423 active. Have you noticed that on the +423 you receive a sms when someone tries to get hold of you and does not leave a message on your voicemail? Also voicemail works much better than in the past (to the extent that I am sorry that now that it's working so well they switch to +447937!