OK, this may be a newb question, but since I'm a newb I guess I'll ask it:

Can scriptmaker adjust the options in a field definition?

I have 3 files, 2 of which have similar "head" information (ie: book title, publisher, etc.) with each book having a unique title ID.

My 3rd file currently does the lookup from file "1", based on TitleID, but because of the age of some of the records, it would be better to lookup from file "2". Ideally, I would like to create a script that changes the lookup definition for each field from "1" to "2" and grabs that information.

is this possible for FM Pro 5.5?

No. But you can have it auto-entered with file 1's data and use a script to overwrite it with file 2's data.

Rather than auto-enter a looked-up value, you might experiment with setting the fields to auto-enter a calculated value (if you upgrade to FM7, try the Lookup or LookupNext functions). Or, since you're using a script, don't bother at all with auto-entry, just set the fields with your script:

#(pseudo logic)

If record is old

. set field to info from file 1

else

. set field to info from file 2

End If