Difference between 'yet' and 'although'

Training for a TOEIC exam, I got this question.

Answers just and moreover are easily discarded.
The choices yet and although remain.

The correct (expected) answer is the first one.
I'm confused why.

Definitions:

• although: in spite of the fact that; even though
• yet: 1. up until the present or a specified or implied time; by now or then 2. still; even 3. in spite of that; nevertheless

According to this question about the difference between 'but' and 'yet', yet might be used when you have an element of surprise.
Is this true and the case here? Can you give me a definition showing this?

If the question had been:
_________, SO FAR, very few customers upgraded their accounts., I would have picked yet.

What makes the answer yet more correct than although?
Could the answer although be considered valid english (even if a "less" correct answer)?

In this context, "yet" expresses a contrast between an implicit expectation and the factual result and signals the latter: "The new services was expected to be a success so customers could have gained some profit by upgrading their accounts. Yet (but/however) they didn't". You could also use "although", but the latter refers to the expectation and not to the result. So you would have to phrase it as: "Although the new service was expected [...], very few customers [...]".