I wish we'd done this at 6 months.
We called DD2 shortversion of her name from the start. DH wanted her to have longversion on birth certificate, something I was unsure of and he later regretted.
Yes, you can be "known as" and that's what we did for some years. Mainly we just told people her name was shortversion and that was that. But it's still problematical, with teachers who insisted on calling her longversion, passports where "known as" is just not good enough, doctors and dentists who repeatedly ignored requests to call her shortversion.
By the age of 12 (and way before) she was very clear she didn't want longversion at all. Every time she told people her name they said "oooh, are you really a longversion?" she wanted to say no - because she didn't feel she was - but that wasn't truthful. We also got to the age where GCSEs, official certificates etc were on the horizon, and she was adamant she didn't want them saying longversion all her life.
So, at the age of 12, we changed it by deed poll. In theory she is still a minor so the only people who had to agree to it were DH and me, but we wouldn't have done it without being led by her. Since doing it she has been so happy with her name. She feels shortversion is really hers now. When people ask if her real name is longversion, she says no and smiles :)
YAB a bit U. They could be saving the child a lot of angst. If parents regret and dislike a name, that will rub off on the child. Besides, the child can always change it back in later years.