Has there ever been any litigation to compel a DB to DC conversion?

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I don't know of anything. Plan design decisions are "settlor" rather than fiduciary functions and therefore no participant could bring such a case on fiduciary grounds allegeing that a DC plan would be more in the interests of p's and b's. If an employer agreed to make the conversion and then did not, there might be some type of misrepresentation claim under the Supreme Court's Varity decision or some type of qasi-contractual claim.

Highly unlikely. As noted in the previous post, the selection of a db plan falls within the non-fiduciary or corporate function and is largely immune from scrutiny under ERISA. Hence, you cannot compel a conversion.