Do you just cut off the old plug and strip new or is there any reason to try to keep the length ?
Yep. Just take the cutters and nip it off right at the plug or, if the plug is gone, just even the ends up. Strip about 1/4" of the outer insulation of (not the individual wire insulation!) and crimp a new one on.
If you're a Digitrax person then, yes, a crimper and extra plugs are nearly essential at some point.