Can I change the custom field values?

Yes, with the following caveats:

a) It's best to modify impacts, probabilities, and risk levels (exposures) by clicking in the Risk Model view. That ensures that the residual variants of those values are kept in sync; and

b) You can rename the options for the Treatment custom field, but if you rename "Avoid", or "Mitigate", or "Transfer" to something else, then you won't see warnings in the Issue View about missing treatment plans. That's because of this bit of code:

     * Determines whether this treatment must be accompanied by a treatment plan.
     * <p>
     * A treatment plan is required if the treatment is to Avoid, Mitigate, or Transfer the risk.
     * @return true if this treatment requires a plan; false otherwise
    public boolean requiresPlan() {
                displayName.equalsIgnoreCase("avoid") ||
                displayName.equalsIgnoreCase("mitigate") ||