My point is that their change of opinion may have been based on various factors, not just on the arrival of new “concrete evidence”. For example, they may have downgraded their assessment of the credibility of sources they were relying on, e.g. certain scientists.

No it doesn't. "Low confidence" in the intelligence community is a term of art, contrasting with estimate probability. Confidence can be low for many reasons, including unreliable sources. For example, this is how it is explained in a National Intelligence Estimate
www.dni.gov/files/docume...