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The reason it can't update from beta12 is likely because you're missing some additional files.
If Bossa doesn't get you sorted check this out:
https://duet3d.dozuki.com/Wiki/Getting_Started_With_Duet_3#Section_Updating_Duet_3_from_RRF3_0_Beta_12_to_3_0_Final_via_DWC
It needs a wide and sturdy gantry to deal with the forces from the longer Y belt, but then it's about on par with corexy.