We have disembarked early from a cruise with no problem with approval before sail date. You can also join late.

What cruise lines have NOT let us do was to skip ports intentionally and rejoin while leaving our stuff onboard.

We wanted to leave the boat in Invergordon Scotland and take a train to edinburgh and get back on the boat.
There may/may not have been a day at sea between the two ports. Its a security issue somehow.