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I’ve got a MacBook Pro Retina A1398 (2,6 GHz / 16GB), and the logic board is damaged. As this computer is not my main computer any more and in order not to spend too much money and as it is, is it ok to downgrade and replace it with an 2,3 GHz / 8GB board instead? Thanks

Not a problem, logic board have the same shape and connectors. However, unless you have a damaged CPU or graphic card it might be worth considering getting your present logic board fixed at component level.