A long time ago I worked on a similar project. In our case we had a ROM with (say) v1.00 of firmware on board. Newer firmware versions were loaded from a master controller into the 8051's external SRAM chip. This is obviously in the old days with separate 64k ROM and 64k SRAM chips. Under command from the master, the 8051 would load the SRAM with firmware (say) v2.00, verify the integrity of the firmware, then switch over to it.

Anyway, the switchover was done in two steps and it used a 2 bit external latch to control the memory access signals. The memory map was flipped over 32k at a time. Sorry I don't recall the logic we used and I don't have a schematic anymore.

The first step (call it flip) involved executing out of the bottom half of the 32k ROM, moving the top 32k of SRAM into code space and then jumping to a fixed address at the top of that SRAM. The second step (call it flop) ran from that fixed location at the top of code space (now SRAM). It would move the rest of the SRAM into the code space bottom 32k. Execution would then restart at 0000.

It was also possible to revert to ROM.

Not sure this helps anyone, but we had no problems with it and it worked for many years. Then Flash came along...