The only limitation when you are no longer on a HDHP is that you can no longer make contributions to your HSA. You can still do everything else with it, including leaving it to grow and tracking new expenses to eventually redeem to get it back out.
There are some HSA custodians that might start trying to make life difficult for you if you are no longer able to make contributions, however. That's on them though, the government doesn't care if you are still on an HDHP or not other than not allowing new contributions to it. If your custodian has extra fees or whatever associated with keeping the HSA with them when you can no longer make contributions, you should just transfer it to another custodian (which is also perfectly fine to do, from the government's perspective, after you are no longer eligible to make contributions).