Can I still spend my HSA money after I switch off a high-deductible health plan?

Yes. Once money is in your HSA, it is yours forever regardless of what health plan you have later. Losing HDHP eligibility only stops new contributions; it never affects the balance you already saved. You can keep spending it tax-free on qualified medical expenses for the rest of your life, and after age 65 you can withdraw for any purpose paying just ordinary income tax, like a traditional IRA.

This is why many people treat the HSA as a long-term investment account: pay current medical bills out of pocket, invest the HSA, and reimburse yourself years later using saved receipts. There is no deadline to reimburse an old expense as long as it was incurred after you opened the account.