Java 8 will be out soon and people will most likely upgrade to it, will this cause a problem? People using Java 6 haven't had much luck running the client so It seems like Java 7 is needed that's why I am asking this question.
Of course I'm not familiar with the actual code (noone but Dan is), so I can only answer this from my experience with java applications in general.
The java runtime is (usually) upward compatible. Java 6 lacks certain features and can therefore not be used for eMunie. Java 8 OTOH should contain everything that's already in java 7, so I wouldn't expect problems there.
Database access in java is normally handled through database drivers using a standardized API. That means you can (usually) update the actual database without changing the drivers, and you can update the drivers without modifying the application using them. For example, as a beta tester I know that eMunie does not depend on a specific version of MySQL. I've even used it with MariaDB instead of MySQL.