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I have a A1784 iPhone 7 plus and I need to replace the logic board While I’m at it I was thinking of buying a 128gb logic board to upgrade instead of the 32gb which is what my phone originally had. If I did replace the 32gb one to 128gb would it be compatible? Would it be compatible with the 256gb one? Thanks in advance

Yes its possible, dont forget to transfer the home button also, it is paired to the logic board.