You can stop paying Swisscom your land-line charges and have them taken over by your fixed-line provider, but monthly line rental fee still applies. This has been possible for the past year or so. For a standard land-line, the fees are CHF 25.25 per month.
You make the contract with your current fixed-line provider and they arrange everything for you. Swisscom doesn't need to be notified and won't disconnect your line. Since you are already calling through pre-selection with another provider, you shouldn't have any contract obligations with Swisscom.
What isn't straightforward is stopping Swisscom bills. We still get small bills every now and again for business numbers