Getting the resplendent hero gives you a neutral 5* copy of them and turns all other copies on your account (and any you might pull in future) into the resplendent version as well. This persists after the FEH pass expires too, so merging wouldn't lose anything. You can revert them to their original art if you want, but they'll keep the stats regardless.
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"Eet's game time..."
https://soundcloud.com/luissera/intro/s-VvkMJ