"The 'sound arguable basis' standard means that changes to health insurance plans as well as other benefit plans will be found unlawful only if an employer cannot plausibly assert that its actions are permissible under the parties' collective bargaining agreement. In practical terms, this means that when negotiating agreements employers should: [1] seek language that affords flexibility and allows modifications to benefit plans; [2] avoid provisions that require union approval for any changes or only after bargaining with the union or that state that any changes to be 'equal or better' than those presently offered; [3] attempt to limit the employer's obligation to one of notification only."