Are there any harm reduction education sites that focus on health implications for adults thinking about switching or quitting?

Yes — there are several evidence-based harm reduction / quitting education sites that focus on the health implications for adults who are thinking about switching away from a product, cutting down, or quitting. The best choice depends on what someone is using, but here are good general options:

General quitting / switching education

• CDC Tobacco & Nicotine
  Evidence-based info on cigarettes, e-cigarettes, smokeless tobacco, and quitting.
  https://www.cdc.gov/tobacco/

• NCI Smokefree.gov
  Adult-focused quitting support and info on what happens to the body when you stop smoking.
  https://smokefree.gov/

• NHS Better Health / Quit Smoking
  Clear, practical health-focused quitting info and support.
  https://www.nhs.uk/better-health/quit-smoking/

If you mean vaping / nicotine

• Truth Initiative
  Strong education resource on nicotine, vaping, and quitting for adults and young adults.
  https://truthinitiative.org/

• American Lung Association
  Health effects and quitting guidance for smoking and vaping.
  https://www.lung.org/

If you mean alcohol

• NIAAA Rethinking Drinking
  Health effects of alcohol use and tools for deciding whether to cut back or quit.
  https://rethinkingdrinking.niaaa.nih.gov/

If you mean cannabis

• SAMHSA
  Substance use education and treatment resources, including cannabis-related harms and recovery.
  https://www.samhsa.gov/

If you tell me what product you mean—for example cigarettes, vaping, cannabis, alcohol, or opioids—I can point you to the most relevant sites and the ones most focused on adult health implications of switching vs quitting.