I have gone through the exact same change in standards that your company did. That is very frustrating that no documentation of rationale or justification was left.
What I did was conduct and document a study showing that:
1) Metal detectors set for the slightly larger standards were still capturing 100% of the metal foreign material detected utilizing the original standard settings. We tested live production on select lines and samples of previous metal detector findings as well as samples of metal detectable bandages, gloves, pens, etc that are used in the facility. -This should have been done before implementing the change, but you can still collect this data. The biggest challenge will most likely be getting a large enough data set before the audit. Expert advice from your MD manufacturer will bolster your conclusion that the current standards meet your needs.
2) Changing the settings for the larger standards produced the desired effect of reducing the number of false positives. -This might not be possible if no one still at the company was aware of what initiated the change or the desired outcome, but talk to people, someone might know more than you'd expect.
1 is definitely more important than 2.