Old number can be kept to cell line in many cases, but new numbers would be new. That will eventually dilute area code identity without too much drama - or what else can you do? Collecting crumbs of non-assigned numbers across the area is only that efficient. So spreading out impact over time is the best bet IMHO.
Can you even collect them by the crumb? At least, the way it used to be is that numbers were assigned to carriers in blocks. That is to say, XYZ Phone Company can't just be assigned 123-555-2010, 123-555-6116, and 123-555-9924 from LMNO Phone Company. Rather, they can be assigned 123-555-2010 through 123-555-2210, minus whatever numbers are already in use somewhere. If a customer drops a number, then that number stays assigned to the same provider until it gets assigned to a new customer with the same provider later. With increased portability, it's possible things have changed, but I'm not sure.
But anyway, I agree that 'area code identity' should be no basis for making area code changes, and that overlays are probably the best system to use.