If someone took a worker’s compensation settlement several years ago, will an Obamacare health plan take over coverage for that injury under the pre-existing clause?
Worker's comp doesn't counts toward ObamaCare subsidy eligibility, as it's not taxable. You can switch to an ObamaCare plan when worker's comp ends.
Worker's compensation is not taxable by the state or federal government, however there is one lone exception: individuals who also receive disability benefits through Social Security disability insurance (SSDI) or Supplemental Security Income (SSI) may have their benefits reduced so that the combined amount of the workers' comp benefits and the disability payments remains below a certain threshold. This is called the workers' compensation offset.
No matter what time of year it is, if a health plan provided by worker's comp ends you can switch to a Marketplace plan with cost assistance via special enrollment (you have a 60 day window once the plan ends).
If worker's compensation is not providing health coverage, and is just reimbursing you for medical costs you may have to wait until next open enrollment to get covered unless you qualify for another special enrollment opportunity. You can always contact HealthCare.Gov for help.