Right now US privacy laws aren't compatible with one overarching centralized healthcare record.

Short of that, however, would be an interoperable system. Epic, which is the largest US medical record system, allows for different facilities on the same platform to share information. It is up to the specific facility if a records release is required. Most systems in a given region will have that worked out ahead of time and build it into their general consent for treatment (a form everyone signs). It works quite well. Where I practice, I am able to get all the information I need from across the country, assuming they are on the same platform at the time I am seeing the patient.

For other platforms, it's more mixed. Federal law requires certain interoperability, but it is fairly limited and not real time. Generally it involves a flash drive with the info on it.

As for the comment about changing platforms in a similar system, that is a struggle. Hospitals are required to keep patient information forever. When they first started going up on electronic systems, they only went back so many years as the scanning costs were huge. As time has moved forward, many systems are bringing all the information over to the new system so they don't have to maintain more than one electronic system for archive purposes.

Source: I am a physician and chief medical officer.