Difference between "I am going to" vs. " I was going to"

In the first sentence that you used: "Ann said that she was going to Las Vegas with her boss, and the whole office was talking about it."

In this sentence, we are actually referring to a past progressive tense. In this sentence, you are talking about an event that started in the past and has already ended.

In the second sentence, you are talking about a future event. You are going to be transferred to Mongolia (It means that you are still in the place where you work but in the coming future you will be transferred to Mongolia).

This certainly shows that the two things are irreplaceable. The phrase "were going" signifies past action while the second sentence is used to refer to coming future.

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