1) You can ask HR, but most employers usually only changed during open enrollment, so you may need to contribute directly
2) If you have a FSA in 2018 and your HSA contributions are all in 2017 (or 'for' 2017), there is not a problem. You can also withdraw from the HSA during 2018 without any problem. You just can't contribute to the HSA in 2018 (unless they are specifically compatible).
If you are switching to a non-high deductible health plan, you (or your employer) can't contribute to the HSA.
If you still had the High Deductible Health Plan, the FSA would need to be designed so that it (1) would ONLY pay out after the high deductible was met, or (2) it would ONLY pay for things like dental and vision.
