The durational clause of a CBA applies to health coverage absent clear affirmative language to the contrary.
Lifetime health benefits for survivors of workers does not imply a similar vesting for workers; in fact, it argues against any ambiguity as to the question where the agreement is silent on that point.
http://www.opn.ca6.uscourts.gov/opinions.pdf/18a0108p-06.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.