On Umbrel you can indeed switch from Bitcoin Core to Bitcoin Knots without resynchronizing. All it takes is dragging your existing data folder into the Knots app.
Here's a quick step-by-step walkthrough if it helps:
Bitcoin is under attack. Switch to #BitcoinKnots.
1. Bitcoin core changes were unsolicited
2. The changes remove freedom to vote with your node for how you want to handle spam.
3. Dissenting informed opinions were censored.
4. The executors have clear financial interest.