Which legacy IT system should be regarded as the master and in which transaction scenarios?

So, "legacy" is not really a valid term in this context, since mainframe systems are the workhorses of IT; current estimates place 80% or more of the world’s current data on mainframes.

If there are many components implemented on the same legacy platform, you can develop reverse transformations from the components to UML.

You may also need to do some capacity planning with them because the extraction needs of your DW could overly stress the legacy data source, thereby requiring upgrades.

For legacy code, rather than looking at individual methods, look at what the application is doing.