57865
So I get the general gist that ことになりました means 'its has been decided that~'

For example:  

> 私は来年大阪に転勤することになりました。

And that ことになっています essential means the same thing, but indicates that the
decision took place at sometime in the past and the result of that is still in
effect.

For example:  

> 日本では車は道の左側を走ることになっています。

But even so, I can't get in my mind around the slight difference in meaning
between:

> 今日山田先生に会うことになっています。
>
> 今日山田先生に会うことになりました。
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2018-04-11T12:36:58.503
57863
2018-04-11T16:38:14.237
2020-06-17T08:18:27.500
-1
29512
question
nuances
ことになった vs ことになっている
The difference in meaning is rather simple.

「ことになりました」 means that the decision/plan/rule has just been made. It may have
been just a few seconds ago or a few days ago. Point is the news is still new
to the speaker.

「ことになっています」 means that the decision/plan/rule was made some time (or a long
time) ago. The news is no longer new to the speaker and s/he is more "ready"
for the event.
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2018-04-11T12:53:50.883
57865
2018-04-11T12:53:50.883
57863
answer