Burn in’s a myth, the closest thing to it would be your brain adjusting to a new sound signature.

Perhaps you just prefer the sound of your older headphones more? If you try the Fidelios for a bit more and still prefer your old ones, there’s no harm in returning the Fidelios and sticking with your old pair, or trying other headphones if you’re still set on getting something new. It’s not like just because something is newer and/or more expensive, it’ll automatically sound “better” to your ears.