I have a REU 1764 with original only 256 KB RAM, but the expansion of
that REU to a REU 1750 with 512 KB RAM is very simple,
just solder 8 DIL sockets in their empty places and insert DRAM 41256-15 (150 ns or faster, doesn't matter), and cut the proper "wire" here:
There is also a resistor "R4", 390 Ohm, which is not used in the REU 1764, but in the REU 1750. Sometimes a C128 has problems with an upgraded REU,
if this resistor is missing (for me it was working without the resistor, but...). See also this link for more information.
The full view of both PCB sides of the REU:
Partial opened (still shielded)
Closed REU (for normal usage)