I would REALLY like to know where the reference to "the standard 8051 schematics" re using a15 is.
I assume you want to change from Harvard (separate code and data memory) to VonNeuman (all memory is the same, as in the PC) configuration.
In addition to using a15 as memory selector you must OR !PSEN and !RD ( negative or, use an AND gate ) in order to make it work.