A cable is a cable, the length does not mean you would need to retune in anyway. Are you planning to cut cable in between the coil and connector and splice back together or take apart the connector and resolder there?
Both would work as long as you resolder and make sure you take a picture so you know what wire goes where.
I have used longer cables and shorter cables on my detectors and as long as you don't make them too long, you wont have much if any loss of signal. kinda like a extension cord after so many feet you loose amps.
Make sure you shield for RF interference if you splice and make it waterproof if you plan on getting it into water.
If you decide to use another shaft you might want to measure and leave a little extra. straight carbon shaft would need more if you customize it.
If you do cut cable, don't throw the extra away! you can use it for many projects down the road. Hope this helps in some way.
Happy Hunting and Happy New Year!