You can definitely lower the TPMS limits. I think the warning (and confusion) that you can only raise the pressure is from a safety perspective. I tested this out, and I was able to lower my rear warning pressure from 80psi to 50psi successfully.
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2011 Ford E350 EB Quigley 4x4 'PUPLGUK' | V10 Gas | Opt Overland Pop Top | GBS LiFeMnPO4 100Ah