[Opinions] Re: How easy is it to start using a new nickname?
It's not impossible, but it can be kinda odd at first, and it definitely takes effort. It's basically helping everyone who knows you learn a new habit, and it's teaching yourself a new habit too, for when you introduce yourself as the name. It can feel really weird in the beginning. I remember when I decided to go by my full name instead of the childhood diminutive I had been called for years, I had to stop myself from giggling whenever I introduced myself as it, because I felt like I was just playing at being a grown-up and everyone could tell.
It will take patience on your behalf if you want your friends and family to change. You'll have to remind them and correct them if you really want to be called by the new nickname. And even then, they might not want to change.
I don't think being known by many names is a bad thing. Especially if they are all as closely related as Danielle, Dani and Elle. I know plenty of people who are known by multiple names. It's an affectionate thing.