Hello,
Changing the dataverse.fqdn and/or dataverse.siteUrl JVM options is NOT going to affect the pre-existing DOIs by itself.
AS LONG AS the urls registered for the old DOIs are still resolving. (dataverse.siteUrl is the *preferred* url for your Dataverse installation; but it's perfectly ok to have multiple URLs!)
However, if the URL registered with any of your old DOIs really becomes unreachable, you will have to re-register the affected DOIs.
For example, you have a DOI registered to redirect to
https://dataverse.uofX.edu/, but the installation has been moved to
https://dataverse.uofY.edu, and the old location is no longer valid. Then, once dataverse.siteUrl has been changed to the new url, you'll need to re-register the DOI, using the API, to update the location in their database.
best,
-Leonid Andreev
Dataverse Project