In general, you cannot MODIFY a UL listed device. The UL listing applies to the unit as a whole. The exception would be if a manufacturer applied for a UL cert for their mod. kit. Then UL would test the mod. This may be the case with your CH kit. A similar situation exists in plumbing: a commercial electric water heater can be configured for almost any combination of volts, watts, phases, simultaneuos/not. But that CANNOT be done in the field. It must be done in the UL certified location. Most manufacturers have several distribution points around the country, and those locations are listed to do the work, so you don't have to wait for a model to ship up from mexico to get just what you need. REPAIRING a water heater with authorized parts, does not constitute modification. Just as REPAIRING your panel with authorized parts MAY be OK.
What is the cost of not having permits? What is the value of your home if it burns down and the insurance company denies the claim due to unpermitted work? What is the value of trying to sell your house sometime in the future, and the buyers home inspector raises that permit flag? Sale delayed, major penalties to the city for after the fact permit, etc.