Employer's contract with Union requires that Employer can change health insurance carriers but must mainatin "coverage at least equal to" the coverage that is presently in effect. Employer provides traditional coverage with a network of partcipating providers. Union alleges that Employer violated contract when it changed carriers because new carrier's network not identical to old carrier's network, and thus, some employees lost the ability to see their providers on an in network basis. Employer maintains that "coverage' doe snot refer to networks but to benefits, deductibles, etc. Is anyone aware of any helpful authorties for the Employer?

IMHO the employer is right. Coverage is what illnesses etc will be treated, while the provider network relates to availability of services. They are probably defined in the PD and the insurance contract.

My question is why did the union sign the Memo of Understanding (or whatever was used) accepting the change without addressing the provider network issue?

It should not be too difficult to expand the network especially if this is a large enough group. Has this been considered?

I do know of an authority that may not be helpful.
First, though, just because the network coverage is different does not mean it is less than the other network. And, even if it was, maybe the in or out of network coverage can have higher percentages of coverage to help make up the difference.
Your example involves a union, so I do not know how the Supreme Court case of Kentucky Assn. of Health Plans v. Miller of April, 2003 would affect it.
This case affirmed that any willing providers laws are legal, for providers cannot be discriminated against, if wlling to accept the same terms of those in the network.
"No longer may Kentucky insureds seek insurance from a closed network of health-care providers in exchange for a lower premium. The AWP provision substantially affects the type of risk pooling arrangements that insurers may offer."
Don Levit

The Kentucky case should be helpful since many other jurisdictions are either of similar opinion or leaning.

That is 1 of the reasons why I stated that it should not be too difficult to add these providers to the network, providing of course that these providers are willing.