After bootup, executing all the initialization routines in the onchip flash it jumps to the external code. the same code that was written in the NVRAM.
Not a good idea, many derivatives either do not react or react rather strangely when !EA is changed after reset.

During RUN time can I move in and out on AT89S53 (code memory). Will the above mentioned circuit serve my purpose.

Or are there any other better methods.
to convert from Harvard to vonNeuman you must negative OR !PSEN and !RD, nothing else is required.

If you REALLY want to do this, I recommend you use an external ROM/Flash so you do not have to change !EA while running.

I just did a quick check od the datasheet for the AT89C53 and here it is:

"EA must be strapped to GND in
order to enable the device to fetch code from external program memory locations starting at 0000H up to FFFFH. Note, however, that if lock bit 1 is programmed, EA will be internally latched on reset."

ERrik