Yeah it should be fine, and its your dog so why not i say

I think after 6 months its more of a problem and some will tell you you shouldn't and some will tell you its still fine. But before 6 months it should be ok.

Yeah shouldn't be a problem. A name that rhymes with his current name will make it easier for him but its your pup so i dont see why you shouldn't choose exactly the name you want.

Also, you can use his new name straight after his old name when you call him, then as he gets used to it more drop the old name and carry on using the new one.

It's really easy to change a dogs name.
When the dog isn't looking at you, say the new name and when he looks, treat.
Or say the new name before the curent name. The dog learns that the new name means th curent name will follow and will start to respond to the new name.

When a stray dog turns up in the rescue, it can't tell them its name so gets a new one, and goes to its new home 7days+ with its new name so it can't be that hard