There is much to know about electrical wiring. Doing a panel swap, which I am guessing is changing from an old fuse system to a circuit breaker system could be a problem. Because you have fairly new wire the insulation
should be in good condition. The old wire should probably be replaced for many reasons. But to do a good, safe and clean job is always first.
The most likely problem you will run into is the wire is just a little too short to reach the breaker. Stretching it is usually not possible; so you now need to decide whether adding a few inches is a good idea or replacing the run is what you have to do.
Sitting down with the code book is a good start. If you do not understand that you need to learn more.
The importance of knowing what you are doing with electricity can save you and your boat. This was demonstrated on a friend's boat. He was wrapping up a few project
things before we were getting ready to shove off for a day of sailing. He plugged in his GPS
to a 12v dc outlet. Nothing happened. Below it worked, topside nothing. But, then we smelled smoke. Smoke and flames were coming from the 12v outlet. Grabbing a screw driver he unscrewed the socket and the wires were on fire.They had shorted and caused a fire. With speed we got that fire out and the socket fell out onto wet paper towels. Whew, that was very close.
Then we saw more smoke and lifted the locker lid to look at the battery
compartment. More wire was on fire, this time at the batteries. Fire extinguishers were used to fight that fire.
With needle nose pliers and a screw driver more wire was pulled out. We felt the wire run from the batteries and most areas were not hot or affected by the melting wires.
Here is what the problems were. First, someone had installed a cigarette lighter style 12v dc outlet in the cockpit
; it was a car/truck/rv unit, not something designed for exposure to the elements in a marine environment
. It was also light duty, not designed to be plugged into and pulled around in a sailboat.
Second, the wires were strung incorrectly from the battery
attachment up to the 12v outlet.
Third, and absolutely the most dangerous of the mess, there was no fuse. Fourth, the wires were bare and shoved into the the + and - posts of the battery terminals.
If we had not been right there when this failed he most likely would have had his boat burn. It might have been too late if we were sitting on the dock
or anywhere else on the boat.
The post mortum. The cigarette lighter failed when he plugged in the dc adaptor for his GPS
unit; it shorted out when the + and - parts
touched each other after many times of use wore through the plastic case. That short caused the wires to over heat and start on fire. There was no fuse on the wire to break a short. The hot wires at the battery melted there and caused the second fire, after the first fire was extinguished.
The lack of a $1 fuse holder and $1 fuse almost burned a beautiful 38 ft sailboat up.
That is why you need to know what you are doing when doing electrical work.