27cXXX eprom switcher

to

In between chip enable or output enable cycles, you could set or reset the
high
order address line.  This would effectively bank switch the EPROM.

I still would not do it on the fly however, I'd feel better if it were
powered off.

you will

you must then switch the highest address line (A14 for a 256) high for
one map, and low for the other.  in practice, a 1k resistor going to
+5 to A14, and then another from A14 to a switch that goes to ground.
a 220 ohm resistor should be connected in series with this switch to
protect the input.  (If its a really noisy electrical environment such
as a car, and the switch is going to be mounted some distance away
such as on the dashboard, then you should consider using an
optocoupler near the ROM to do this job and avoid electrical
interference possibly getting into the A14 line via the length of wire
that goes to the switch.).


Switching a ROM (or any device thats connected to a processor Address
Data bus like this such as RAM etc) while the software is running
isn't a really good idea, especially if there are changes to the
operation of the actual CODE running in both roms (as compared to
changes in data tables within the ROMS).

When changing software versions, its often the case that the program
will Clear any NV RAM back to default settings, and if there are any
changes that you have to make (ie, settings you need to enter for your
particular options that are then stored in an internal NV RAM).  If
the program learns and calibrates itself over time as the engine is
used, then all this special (and probably constantly changing) data
will likely be lost too, leading to less than optimum performance
until this data is replaced.

At worst, engine damage may occur, or it might stall, or might fail
completely (from thinking the computer is stuffed, and shutting down
to protect the engine, or for safety reasons) needing a service centre
to reprogram the settings etc.  At best, you might generate many ROM
checksum errors in the system :)