I changed my name legally 5 months before I started using it full time. I told people that my legal name was Dennis and if they felt comfortable using it, that was cool, but it was not going to be an option after September (this was April).
I suggested Den as an alternative, which some people still use, including people who didn't know me before. Guess it's just a natural derivative of Dennis. Some people who knew me before felt more comfortable changing to Den. And actually, it feels more comfortable to me, tbh, notwithstanding that I've been Dennis legally for more than a year and socially for 8 months.
And ID-wise, I waited till after chest surgery to change it because I was travelling to the US for surgery and post-Bush, they've been awfully weird about ID, so I thought if all else failed, I'd travel south as female and north as male. Which is what I did. I had a little trouble passing going south and none whatsoever going north, even though my ID didn't match my gender presentation. They all just looked at it and said 'thank you sir' and sent me on my way. It was a bit of a struggle presenting as female going south probably because I was SO uncomfortable with it.
Dennis