Travlr37-
The response to an EPIRB or a PLB is basically the same. The only major differences are:
A PLB is registered to a person, an EPIRB to a specific boat
A PLB has a shorter run time, 24 hours IIRC, vs. 48 hours for an EPIRB
A PLB generally needs to be held clear of the water to work properly, an EPIRB will float and work just fine
A PLB doesn't usually have an integrated strobe, the EPIRB does.
As for taking the EPIRB off of a boat and using it on another boat without changing the registration info—it is a really bad idea.
For one, the registration information has contact information in it that is often called when the EPIRB goes off. If the person answering the contact information doesn't realize what's going on, they may call the SAR effort off before it even gets started... imagine... they get a call at 4:00 am. They're asked about the boat, and they reply, "It's sitting at the dock, I was on it last night, and everything was fine." What do you think is going to happen with the SAR efforts... nothing.... they're shut down.
Second, the EPIRB registration has a vessel description. If you borrowed the EPIRB from a blue Tartan 37, and you're now on a 65' red Swan... they're going to be looking for the wrong boat.
Third, the vessel description also says how many people generally are on board... and if you've borrowed it from a 37' boat with a crew of two, and you're on a 65' boat with a crew of seven, they might not have the capacity to rescue everybody aboard.
Fourth, they may try contacting the boat in distress using the MMSI, or via name. Since you're not on the boat in the registration, the name and the MMSI will be wrong.
Finally, it may be taken as creating a false distress call, since the boat that the EPIRB is registered to isn't actually in distress more likely than not when you trigger it.