Erin's name being Kelly makes no sense

We all know that Erin's real name is Kelly, and 'Erin' is just her middle name.

So besides that one episode, we never see her addressed as 'Kelly' any more than we see this name thing addressed during the whole show.

Now, because she goes by Erin everywhere she goes, with everyone she meets, even with her birthparents and all..

..when, how and who would ever have started to call her Kelly in the beginning anyway?

I mean, if she loves being called 'Erin' so much, why would she ever have introduced herself as Kelly in this ONE instance only, and nevermore since?

Why would everyone in the whole office think her name is Kelly until Charles puts an end to it, and why would that one thing dictate her WHOLE FUTURE with that name change anyway?

Furthermore, she says something like 'If we are CHANGING names' (emphasis mine), which implies that she, indeed, HAS gone by Kelly, or at least something other than 'Erin' since that point. Otherwise it wouldn't be 'changing' anything, now would it?

So this 'Kelly in only one episode' thing makes no sense - she should either have been Erin from the beginning, and no one should even know her first name is supposed to be 'Kelly', or her first name would show up at least from time to time, from people outside the office, and maybe even from Creed or others who might not have paid attention to this whole 'name change' thing during that short period of time during one day.

She should either have only been known as 'Erin' from the beginning, or she should have to be referred as 'Kelly' at least SOMETIMES, because this kind of sudden name change isn't instant, perfect and all-permiating. There might even be people that REFUSE to call her 'Erin', since they have always called her Kelly. I guess writers don't think about this type of things.

I once knew a female that had a 'romantic' middle name, and very 'mundane' first name. You can guess they went with the first name only for awhile, then wanted everyone to use their middle name after a certain point. I resented this, because I had always used her first name, so even though she was a good friend, I never called her by her middle name.

Then there was someone's half-sister with the exact same situation and behaviour - people often want others to call them by their more 'romantic' or 'classy' middle name, when the first name is mundane or even ugly. That's how I know that not all people will EVER accept such a sudden name change, if they have always used one name. They are not going to start using another just because someone decides so.

I mean, would it REALLY have been too much to write a couple lines here and there peppered throughout a few seasons? "Hey, Kelly, you have a package." "Ah, I go by Erin now, but thanks".

Or maybe 'My name is Erin now, because there's already a Kelly at work'. 'Oh, well, I will still call you Kelly - after all, I don't work in your office'.

Or maybe even 'Yeah, that's not gonna happen, I will still keep calling you Kelly'.

That would've at least been realistic and made it make some sort of sense, almost.

As it is now, the whole name hassle makes absolutely no sense.