I changed a few engines at sea, it was pretty uncommon in the latter part of the Sea King's life. It could be a finicky job and as SKT points out, the high speed shaft could give you some trouble from the VA side. The forward mount required a Non Destructive Testing inspection while the engine was removed which demanded silver soldering skills should the mount bushing need replacing. Those skills were not available on the dets at the time, current 148 dets sail with an ACS tech. We used to have to lean on the Hull Techs for fabrication or structural skill-sets, they did great work and they usually got a kick out of working on the helo.
Main rotor blade changes are a much bigger pain, I was unlucky/fortunate enough to have to do a few while sailing. Same issue with ground run/test flight requirements. Adjustments can be a pain and it really disrupts the ship's daily operations to have a spread helo on the deck or to be obligated to keep returning to a flying course while underway.
This recent engine change on the 148 was an unfortunate requirement, they've been pretty solid thus far. The whole engine needed to come out.
