IBM have an article about moving from VisualAge for Java to Websphere Studio Application Developer.

The disadvantage of having a native JRE, rather than the UVM, is that some features of the UVM are lost. The most noticeable one is "hot swap," the ability to make changes to code and have the UVM load the changes without having to exit the program. This is a huge productivity aid in VisualAge for Java because while you are debugging a program, you can make changes and then test them immediately. You can also rewind the stack frame inside a running program, make changes to variables or run ad-hoc code inside the program, and then resume the program. This feature is proprietary to VisualAge for Java's UVM, although it is included in JDK 1.4. Application Developer was developed while the JDK was still in beta version, so although JDK 1.4 is supported as a run-time environment, the Application Developer debugger does not support hot swap.

I have a feeling that this is one feature of VAJ that I'm really, really, really going to miss. The whole "Something's gone wrong, fix it and drop the stack frame" thing was awfully convenient.