Use of "already" in context or alternative phrasing

On a blog post I (German native speaker) recently wrote the following sentence:

I forgot to tell my boss that the meeting will start already at 2 tomorrow.

Someone then posted a comment that the already doesn't make sense and wouldn't be used by a native speaker of English, correcting my sentence to:

I forgot to tell my boss that the meeting will start at 2 tomorrow.

Now, adding already to the sentence (in German) does one of the following 2 things:

Expressing that the meeting, which was originally scheduled to start at 3, has been "preponed".

or

Expressing that the starting time, yet unknown to the boss, is early from the perspective of the boss/ myself because maybe usually meetings start at 5 or maybe we are about to work till 5 at night today so 2 tomorrow is early for us.

If not using already, how would either case be expressed in English then?

To the best of my knowledge, English does not have a similarly concise indication of a schedule change. Most likely, if they wanted to emphasize the "preponement", a native speaker would say something along the lines of

I forgot to tell my boss that the meeting tomorrow has been moved up to 2:00.

If the correct meeting time has not actually moved, but they misread the schedule or made a bad assumption about it, the native speaker might say

I forgot to tell my boss that the meeting tomorrow is actually at 2:00.

As you can see, there's no indication of whether the meeting was moved up or moved back in this sentence; the most obvious way to give that indication that comes to mind for me is to simply state the originally-thought time, thus:

I forgot to tell my boss that the meeting tomorrow is actually at 2:00, not 5:00.

I forgot to tell my boss that the meeting tomorrow is actually at 2:00 instead of 5:00.

And then leave it to the listener to determine the direction of the move.

If the time has been changed, the usual form is "The meeting will now be at 2 pm tomorrow."

Your second possibility is slightly unclear, but 'maybe' you want "The meeting, unusually, will start at 2 pm", or "The meeting will start at 2pm (sic)". Neither of these is common.

And yes, already is not merely unusual for a native speaker; it is so illogical in this context that it is wrong (IMO, obviously).

I understand your confusion. According to the Oxford English Dictionary (OED) already can mean:

already, adj. and adv.

Before the time in question, beforehand; by now; as early or as soon as this.

But it’s not correct in the context you’re using it in. Correct use would be:

Would you like some help preparing dinner? Don’t worry, it’s already prepared.

In other words, it’s been prepared prior to the question being asked – before the time in question.

In your instance you would be far better simply stating the meeting has been brought forward from the time your boss would have usually expected it to take place:

I forgot to tell my boss that the meeting has been brought forward, and will now start at 2pm tomorrow.