Sorry, this would be in the US. Grandpa Irving ???? was born ~1925 in New York or New Jersey, not sure which. I assume Social Security and IRS would be involved with a name change, as well the state. My sister legally changed her first name from Cathy to Kathi, and I think she had to go to the county seat to do it. After I was divorced, I also had to file papers to drop the ex's surname which I rarely used.
I don't believe you can just switch from Jones to Smith and back again willy-nilly, I think you have a "legal name" for signing legal paperwork. Margaret can use "Peggy" every day & have it on her lapel card, but she can't get married w/ Peggy or file her income tax that way. Irving can't just decide he likes the name "Berlin" & begin presenting himself w/ that name unless he jumps thru some hoops, right?