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Hi. I need to change my logic board (i5 4gb ram) because it is only charging with 2W. To a used one from ebay (i7 8gb ram). The SSD is encrypted. Can I still use the same SSD without formatting it?

Before you alter anything why don’t you just make a clean full backup unencrypted. If you used Apple’s FileVault2 encryption you should be fine. I would still have a master password and a recovery key setup beforehand. If you used someone else’s encryption tools you’ll need to ask them.