If one participates in a high deductible health plan and health savings account, then later transitions to a normal health plan that does not qualify for HSA what happens to the account? Are the funds forfeit? Can you still use the funds for medical expenses? Can the funds sit in the account gaining interest forever?
The HDHP requirement applies to your HSA contributions, but not to your HSA distributions.
Once you have money in your HSA, it is yours to spend on qualified medical expenses, whether or not you remain eligible to contribute further. You can pay for any out-of-pocket medical expenses (discussed in IRS Pub 502) that were incurred on or after the date that you opened your HSA.
The money in your HSA never expires (unlike an FSA) and is yours to keep until you spend it.