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Many questions. How much Googling have you done?
> Is there any upgrade program or something similar?
No.
> What do I do with my old device when I upgrade to the new model?
Whatever you like with it.
> In terms of security, speed, etc., which one is better?
The security is the same. It has a processer that is a million times less powerful than your mobile phone, are you thinking to over clock it?
> I choose to get paid in crypto, does my addy change, and do I need to give a new one each time I get paid, or does it stay the same?
That is up to you. In Bitcoin, it considered best practise not to reuse addresses. Again, did you try and look anything up?
> Thanks in advance!
I doubt anyone who writes this really means it.