We have a system that uses OTP (one-time programmable) EPROMs, specifically the AT27C256. We would like to replace this EPROM with an EEPROM or Flash chip such that we can make our units field upgradeable, more serviceable, etc. Obviously, we want this change to require little to no hardware modifications. These EPROMS are currently the code storage for an 68HCP11 micro.
It looks like the AT29C256 would be a viable option, as we could use it with only one pin change (change Vpp to WE_N). However, it appears that these chips are not made any more and sourcing is very unreliable, especially as we need ~70K of them. As such, we're looking at any other reasonable options.
We've considered adding a daughter board that sits on our EPROM's footprint (PLCC32) and contains another flash chip (AT28 series?) along with whatever conversions are necessary to make it work with our HC11 micro. We've also considered adding an FPGA to the daughter board between the micro and flash such that a series of "magic" reads can perform writes to the chip, but this seems to be getting a bit excessive for a problem many others must have solved. It would, however, allow us to not use jumper wires on our existing boards.
What have others done in this situation?