Per Department of Labor and ERISA, a company cannot change what you have vested and cannot force you to take a lump sum for what has happened to date. https://www.dol.gov/agencies/ebsa/about-ebsa/our-activities/resource-center/faqs/retirement-plans-and-erisa-consumer
"Can a plan reduce promised benefits?
Defined benefit plans may change the rate at which you earn future benefits but cannot reduce the amount of benefits you have already accumulated. For example, a plan that accrues benefits at the rate of $5 a month for years of service through 2010 may be amended to provide that for years of service beginning in 2011 benefits will be credited at the rate of $4 per month. Plans that make a significant reduction in the rate at which benefits accumulate must provide you with written notice generally at least 45 days before the change goes into effect."
They can OFFER a lump sum, and they do, and they might even offer more than the current default shown at the retirement site, but you don't have to take it. See https://www.kitces.com/blog/irs-notice-2015-49-cracks-down-on-accelerated-lump-sum-pension-buyout-offers/