You keep your HSA and the $ in it when you leave. You can use it any time. You might have (small) fees associated with account admin that your employer was paying depending on who is administrating the HSA--that said, I checked my Fidelity account and they have no fees, so not sure how common that is.
- You can only make additional contributions to the HSA while you're covered by a HDHP.
- If you extend your current HDHP with COBRA, you can continue contributing to the HSA. If you have no coverage, you can't continue to contribute--you can still use your existing $.
- There's no restriction on joining another low-deductible plan. If you join/switch to a low-deductible plan, you can't continue to contribute to the HSA--you can still use your existing $.
If you get fired or leave, you generally keep all the funds in your HSA with no clawback. Worth understanding your company's HSA funding mechanism/schedule. If you self-fund the difference (maybe they give you two weeks and you decide to load up), you can do an itemized deduction on your tax return.
- Payroll deductions generally also avoid payroll tax, so if you thought you were immediately going to get a new job and go HDHP again, loading up might not be the best strategy (but the amounts we're talking about saving are in the hundreds at this point).