So I just finished my 2nd msx2+ upgrade. This time a nms8245.
Since it was kinda hard to find these instructions, I will just dump them here 
Installing the v9958
Cut out the v9938, remove the remaining pins and solder.
Solder a socket in place of the v9938
On the v9958 these pins have to be bend so they are not inserted into the socket:
4 (vreset) (Should not be needed as it does not seems to be connected in nms8245...)
26 (wait)
27 (hreset)
21 (vdd)
Then connect these pins together on the v9958:
1, 4 and 27
21 and 58
Installing the ROM
Since there is no F4 port, make sure the F4 port stuff is disabled in the BIOS.
Create a 27c512 or similar system rom with this layout:
0000-7fff basic-bios
8000-bfff subrom
c000-ffff diskrom
For the kanji rom I've used a 27c512 but a 27c256 should also work.
It should have this layout (for a 27c256 just skip the 2nd half):
0000-3fff kanji rom 2nd 16k block
4000-7fff kanji rom 1st 16k block
8000-bfff kanji rom 2nd 16k block
c000-ffff kanji rom 1st 16k block
Insert the system rom in the rom socket.
Bend pin 27, 20 and 22 on the kanji rom and install it on top of the system rom.
Connect the bend pins as following:
- pin 27 on A14 i.e. pin 25 cardridge slot
- pin 20 on CS12 i.e. pin 3 cardridge slot
- pin 22 on slt3/30 i.e. pin 48 of the gate array. There is a nice place to solder it right above pin 48.
That's it!
