This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Java class files have no concept of 32 vs. 64 bit - it makes no difference.
Classes compiled with Java 7 can run on earlier JVM versions provided appropriate -source and -target switches were used, AND the code doesn't use Java 7 features or doesn't use them on earlier JVMs. That's show it has always worked.