I'm not aware of any method of changing the current working directory (where the outputs will be saved by default) in 12c. In 11g it was possible to change it - REP-69 REP-56133 Using Destype=FILE Without Desname (Doc ID 1925057.1). What you could try to do is create a library (pll) in which to put the code you use to call reports from forms and attach that to the forms. Then you would only change the code in the pll library.
I was afraid this would be the case.