Well, maybe I don't understand your question exactly. Do you mean between compatibility between LabVIEW version (6.1, 7.0, 7.1, etc.) or version of you program where you change the data structure?
Generally, LabVIEW version are compatible (but I recall some small incpmpatibilities many version ago). If you change the data type of your objects, old files will no longer be compatible. In this case you need to write some conversion routines for backward compatibility. You can e.g. try opening the datalog file using different historical "datalog types" until you get no error, then map the old data into the new structure as needed. Alternatively, you could also use a more generic format such as XML or even plain ASCII.
