IBM provides a utility which will analyze your programs readiness to go to 6.1. If you have the source you should good to go. When we ran the analysis it reported programs which the system will not be able to convert to 6.1. In our case, these were purchased programs for which we have no source.
This is a free utility provided by IBM.
In general, you do not need to recompile programs that were compiled under version 5. Programs that were compiled before version 5 should be recompiled if they do not have observability.
IBM changed how observability worked in version 5.
Before version 5, you could remove all observability — this would stop the conversion. As of version 5, there is some observability that is not removed — this portion is used by version 6 to do the conversion.
As noted above and in the discussion, the ANZOBJCVN utility will tell you what can not be converted.