No you don't need to downgrade to 1.5. All you have to do is follow the instructions on updating it to 3.40 OE-A and that's it. Just make sure to put the files in either your "GAME" folder or "GAME150" folder which ever one you hav set to 1.5 kernal loading. To check this just go into recovery mode and see which kernal "GAME" folder is set to. Either on 2.71 or 1.5 kernal it should be set on. Depending how you configed it. It doesn't matter which one.

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