John Collins worked amongst the cooks (as a scullion) scrubbing dirty dishes till his hands ached. That doesn't make him a cook in any sense though.
James Johnstone signed on as a saloon steward, wore their uniform and still worked in the first class saloon.
On the day of or on the day before sailing Johnstone and a couple of others were most likely told their duty this voyage was to stand watch at night and make sure there was nobody trying to steal or vandalise anything. Other shipping companies did have specially assigned night watchman but it doesn't appear that White Star bothered with such an official position in 1912 anyway.
Remember, serial spinner of tall tales Frank Prentice claimed he was a pursers clerk fifty years afterwards on radio and television and did so for the remainder of his life. The fact remains he was a humble storekeeper, there is not one iota of proof he was anything else.
During the centenary of the disaster I lost count of how many grandchildren and great grandchildren of crew survivors appeared on UK television or radio and claimed that their grandfather or great grandfather "was the bo'sun on the Titanic and was in charge of the last lifeboat to leave". If we believed them then the Titanic would be listed as having over fifteen bo'suns and they were all in joint command of Collapsible D !
What their job is listed as being on the Particulars of the Agreement is what they were and should remain as it is. Also if their discharge books survive, it will most likely have the exact same listing as the PoA.
So for me, they stay the way they are. No change.