There are a number of factors to consider when migrating to a new version of a Java Virtual Machine. There may be new features supported and others deprecated.
Starting with WebSphere® Application Server V8, you can increase the JVM heap size using the following options on the command line call to WASPreUpgrade and WASPostUpgrade.
-javaoption -Xms <initial heap size> -javaoption -Xmx <maximum heap size>.
For example -javaoption -Xms4096m -javaoption -Xmx4096m