How to soundproof a second story floor

Are you saying that your HOA has no ARC guidelines on hard surface flooring?? Do your documents even permit hard surface flooring? (Some do not.)

> How can one ensure that a contractor is going to do the right thing?

You can require particular products and techniques, with inspection

> to avoid a future lawsuit

but this is America, there is no way to guarantee no future lawsuits.

This topic has come up a few times in the past.

One of the things I found in those earlier threads was that flooring materials have an ASTM impact class rating that helps predict how much noise will transfer to the space below. It appears, however, that the final result is not the sum of the parts as there are many variables beyond the manufacturers' control, including the quality of the structure and the quality of the installation. For this reason, one must bring in an engineer after the job is done to test for sound transfer. Not only does this add to the cost of the installation, if the work fails it must be removed and replaced until it passes. Nobody wants to pay for that.

As Tim suggested, start with a search. If your association has control over the flooring (and it should) you should adopt rules that are consistent with the ASTM specs for sound transfer. I would strongly recommend green-lighting the carpet-to-tile project without some means of ensuring that the sound transfer will not be a burden on the lower-floor unit.