If you have any private .spad files (that is, library files which were not shipped with Axiom) you will need to recompile them. For example, if you wrote the file regress.spad then you should issue )compile regress.spad before trying to use it.
The internal representation of Union has changed. This means that Axiom data saved with Release 1.x may not be readable by this Release. If you cannot recreate the saved data by recomputing in Release 2.0, please contact NAG for assistance.