A neat little trick, i once used and now recall. If you put an XOR in series with the A16 pin and do not allow data storage below the first bytes, and both 0xxxx hex and 1xxxx hex are identical for the first bytes, you can make this so that under a given condition (switch or other) the code in the current program part will load new code to the data part and then switch the signal to the control leg of the XOR.
The first bytes: 0- last ISR call/jump followed by the necessary size to make the startup code read and switch the XOR pin.