In your examples, yes they are interchangeable.
Essentially both sentences state that something happened (she felt happy) once a condition was met (she saw her boyfriend).
However in other situations the two may carry slightly different inferences. There are also other situations where they are not.
Example of different inferences:
We can buy a new house as soon as we sell our old one.
Here, "as soon as" implies that it will happen almost immediately after.
Once we sell our old house we can buy a new one.
Here there is not quite the same implication of immediacy. The condition is there, but it is not explicit that buying the new house will happen immediately.
An example where they are not interchangeable:
Please do this as soon as possible.
This is an instruction to do something at the first available opportunity.
Please do this once possible.
A native English speaker just would not say this. Even if phrased as "please do this once it is possible", it does not sound natural.
You'd more likely hear:
Please do this at once.
This would mean do something immediately. To be clear, "at once" does not mean the same as "once" in your original example.
And of course there is another entirely different meaning of "once" which would be completely confusing if used:
Please do this once.
Which would mean do this only one time.