Correct. Once Magneton has been attached to a Pokemon that already has at least one other Energy attachd to it, you may choose to return it to your hand to fulfill the effect. The card's effect doesn't require you to pick a different Energy card- you certainly can pick Magneton if you so desire. This works because you first attach Magneton, then you need to return an Energy card to your hand. By the time you reach that point, Magneton is already attached as an Energy card, and is treated like one for all purposes while it remains in play.