Problem with converting NATE factories to something else, is that it will be damaged, and have to be repaired, and you need army fighters NOW, not in 2 months.
So, if you convert Maebashi factory into OSCARS, you will make maybe 10 planes during December, but if you leave NATEs in place, you will have 30 (or more, if you expand).
Not, that NATEs are any good (although they seems to work good against unarmored planes). However, you need to destroy as many early Allied planes, as you can, before he rebase them to rear, and wait for reinforcements to employ them again.
So, either few OSCARS, and many NATEs now, or converting SEVERAL factories into OSCARS, to get them immediately.