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The GUID of the datafile can change when running locally (it stays the same when in the "cloud").
I noticed this regularly when updating the datafile between versions.
It doesn't change 100% of the times though as I have noticed that some datfiles will retain the same GUID when upgrading during the last few versions.
When working locally you just need to provide a username and password.
See here for details:
Do I need an API Access Key for Desktop Only applications?
http://myob-technology.github.io/accountright_sample_csharp/ (below) seems to indicate that it is not required.