We deliberately do not do this. The functionality of the two nodes is different in ways that will actually break a program that is not expecting error dialogs to be thrown. We break the code so that you, the developer, have to fix up your code and take this difference in behavior into account. Saving for previous and replacing the nodes would give a misleading run arrow. In general, if we cannot preserve the same behavior during save for previous, we prefer a broken diagram to a fixed diagram with different behavior.
Declined for reasons listed here: http://forums.ni.com/t5/LabVIEW-Idea-Exchange/When-exporting-for-older-versions-automatically-replac...