That doesn’t work, because there is no clear point you could define for a switch between two systems. For example, if you say that Abraham and Isaac’s ages were counted in years, they still lived to be 180 and 175, which are not reasonable to our modern sensibilities. On the other hand, if you say their ages were counted in months, then they died when they were 15 and 14 years old, far too young to have sired as many children as the Bible says they did, much less to do all the other things they Bible says they did AFTER fathering those children.
I forgot to add that Abraham and Isaac didn’t fit into my suggested “system.”
But, I don’t see why the ages in those books couldn’t have been done according to the calendar usage and practice of the different writers, of which there were many, and over a substantial period of time.
I’m not being dogmatic here—it’s just a possibility that occurs to me.