This is a common issue.
So, I made a simple Photoshop action to do this.
Actually, there are two.
One will give you only the high frequency part, and one will give you the mid frequency and above. (Installation: Use the Load Actions… menu, via Photoshop’s Actions panel)
It’s not as good as a re-bake from the very high res. geo to the high res. geo, but much faster.
https://www.dropbox.com/s/xf5pccq3z98l81r/Normal_FrequencySeparation.atn?dl=0
Cheers, D
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