The real SpongeBob only has four lines in "Plankton Retires"after which his robot counterpart speaks for the rest of the episode.
The design of the Robot Plankton is just like the one from 'Sponge Out of Water', while Robot Krabs and Robot SpongeBob are new designs.
'Plankton Retires' is the fifth episode where Squidward appears but doesn't talk. The first is 'Reef Blower', the second is 'The Chaperone', the third is 'Procrastination' and the fourth is 'Wet Painters'.
Although Squidward is heard snoring and exclaiming in "Plankton Retires", Rodger Bumpass isn't credited among the cast of that particular episode because Squidward has no speaking role. Similarly, in "Texas" the octopus is heard yelping in pain, and Bumpass still wasn't credited (later in the episode Squidward says "Whoopty-doo", although it's inaudible). However, in "Whale Watching", Tom Kenny is credited in the episode cast despite the fact that SpongeBob doesn't talk in "Whale Watching", he just laughs.