This is a HUGE deal, especially in California, where an estimated 430,000 residents are enrolled in off-exchange ACA policies which are virtually identical to their on-exchange equivalent, with the sole distinction of those enrolled in them not being eligible for ACA subsidies.
With subsidies being beefed up and the 400% FPL subsidy cliff having been killed (for the next 2 years, at least), this means that hundreds of thousands of Californians have just become eligible for thousands of dollars in savings...as long as they transition to the same plan on-exchange.
In the United States, major medical insurance policies for those who don't have healthcare coverage through their employer, Medicare, Medicaid, CHIP, the Veteran's Administration or some other source are available via the ACA's individual market exchanges. The individual market for residents of 36 states is HealthCare.Gov; the remaining 14 states + DC each have their own ACA exchange, such as Covered California, NY State of Health and so forth.
There are usually dozens of ACA policies available via the ACA exchanges, but they fall into five major categories: Catastrophic, Bronze, Silver, Gold and Platinum plans (other major distinctions include HMOs vs. PPOs and other variables,but those are for another day).
With rare exceptions, Catastrophic plans are only available to enrollees under 30 years old. ACA premium subsidies can't be used to help pay for Catastrophic plans either, so enrollment is rare; during the 2020 Open Enrollment Period, only 89,000 ACA exchange enrollees selected Catastrophic plans out of over 11.4 million total, or just 0.8%.