Actually, things weren't that simple, but not too much complicated neither. The system also has a Programmable Peripheral Interface (82C55A) memory mapped at c000h-c003h, which makes things more difficult.

System memory map when switch is "UP" is as following:
ROM: 0000-1fff
RAM: 8000-bfff
PPI: c000-c003
I have not schematic presently, but the possible decoding can be done this way:
1. A15 nand A14 --> ~PPI
2. A15 or A14 --> ~ROM
3. (~A15) or A14 --> ~RAM
When switch is reversed, all that has to be done is just swapping 2. and 3.

I hope that's finally how this system works.

Ivan