My comments really only can be taken as "true" for the Leroy Sumer units used on boats before 1997. They are the only ones I have had in hand and taken apart. The Italian ones might be different...
That said, the top and bottom covers need to come off. The motor does not need to be removed from the gearbox, unless you have concerns about the worm shaft seal. Since that is fully protected from the environment, I wouldn't expect it to have problems.
Once you have the box dismounted it's a pretty easy job from there. The covers are sealed with o-rings. The lip seals pop out in a second with a seal puller, and new ones just just tap into place. A good chance to check the insides and renew lubricant if it has water in it too!
Check the o-ring seats carefully. One of mine had corrosion pits that would have prevented a good seal, so I used some RTV on that one to fill the gap.
The bit about "permanently lubricated" might be true--but only until a seal fails.
This really isn't high tech mechanical engineering. The only high wear contact point is between the worm and the worm gear. Other than that, it's just four lightly loaded ball bearings.
I think there are two issues with these units. First, is the housing alloy really is marginal for salt water exposure. Second, the top seal is exposed to the sun. The rubber parts do NOT like the continuous UV exposure. A cover when not sailing would never hurt...
On the hard at LMC,
Fort Lauderdale, FL