Basic LAMal is guaranteed, whatever happens
It's a fundamental protection of the Swiss system: no insurer can refuse you basic LAMal, cancel it, or raise your premium because of your health. Whether you're perfectly healthy or have a chronic illness, you pay the same basic premium as everyone and get the same benefits. Illness costs you no rights on the basic insurance.
Choosing your deductible with regular costs
If a chronic illness brings regular, high medical costs each year, the minimum deductible (CHF 300) is usually best: you reach the ceiling quickly and insurance takes over. The reverse logic of a high deductible (for rarely-ill people) doesn't apply here. Recalculate each year based on your foreseeable costs.
Switching insurer and supplementaries
You can freely switch insurer for the basic cover, even with a chronic illness: acceptance is guaranteed, so compare to pay less. Be careful with supplementaries (LCA), though: never cancel an existing one before you're sure to be accepted elsewhere, since on the LCA an insurer can refuse or exclude for history. On the base, compare freely; on supplementaries, keep what you have.