I follow a YouTuber, currently called 'Midway To Mainstreet'. He has changed a couple time and been very successful with his changes. What he did was start adding a some newer content, and see what the reception was.
When I first started watching his vids he was a Disney vlogger, with an ill-fitting name "Rob Plays".
He used to be "Rob Plays". A gaming channel, mostly Minecraft. He started showing some Disney inspired Minecraft areas and they were popular. So he transitioned into more Disney content. ('you like Dinsey Minecraft, here is some of the real park' kind of stuff) The new content proved to be more popular than the gaming vids. He slowly transitioned into just Disney content. Mostly park history. Later, he wanted to go into more history of all parks, boardwalks, traveling shows etc.
So, he did a few vids of the new content, and it caught on. He recently changed the channel name. He is still popular, and still growing. Doing completely different content, he probably lost some of his earlier subscribers, but more than made up for it with new ones.
He was open about what he was doing, and did it in small, well discussed, steps.