User Philip has posted a forumpost on msx.org on how to upgrade a NMS8245 to a MSX2plus.
This blogpost will describe this upgrade step by step supported with pictures and the necessary ROM files to do this upgrade your self.

(thank you Philip for making this post possible)

Since I’m upgrading a NMS8245 that already has a memory upgrade, this first image is for reference only.
For this upgrade I’m using blue solid wire (please ignore all other colored wires)

Step 1 : Remove the mainboard from the NMS8245

Step 2 : Remove the V9938 by cutting all pins and then desolder the pins from the mainboard.
Also remove the heatsink by cutting it directly above the mainboard. You can re-use it for the new VDP.

Step 3 : Place an IC socket on the empty space, where the V9938 is removed. Notice that the little notch in the socket is aligned with the notch printed on the mainboard.
Don’t solder the new V9958 directly on the mainboard.

Step 4 : On the V9958 these pins have to be bend so they are not inserted into the socket:
4 (vreset)
21 (vdd)
26 (wait)
27 (hreset)
Place the V9958 in the socket with Pin1 (the little dot on the V9958) on the side of the notch.

Step 5 : Make the following connections :
Connect Pin 1 – 4 – 27  on the V9958 itself.

Step 6 :
Connect Pin 21 on the V9958 to Pin 58 (I chose to use the printspot 58, so I have don’t have to solder to much on the V9958)

Step 7 : Apply cooling paste on the V9958 and some drops of glue.

Step 8 : Place the heatsink on top of the V9958

Step 9 : Download the necessary Rom’s

Download

NMS8245 MSX2Plus SystemROM + Kanji and Bootlogo 73.93 KB 5 downloads

MSX 2+ System Rom and Kani Rom + Bootlogo for NMS8245 with the following specs: These...

Step 10 : Burn the Systemrom on a 27C512 Eprom.
The Systemrom has the following layout:
0000-7fff basic-bios
8000-bfff subrom
c000-ffff diskrom

Step 11 : Burn the Kanji + Bootlogo Rom on a 27C256 or 27C512 Eprom.
The Kanji + Bootlogo Rom has the following layout:
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
(For a 27C256 skip the 2nd half of this Rom)

I’m using a Wellon VP-280 Eprom burner.
Don’t forget to verify your Rom after burning.

Step 12 : Remove the current Systemrom and Place the new Systemrom in the socket.

Step 13 : Bend the following pins on the Kanji + Bootlogo Rom, so they are not connected:
Pin 20 – 22 and 27

Step 14 : Solder the Kanji + Bootlogo Rom on top of the Systemrom.

Step 15 : Make the following connections:

– 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.

– pin 27 on A14 i.e. pin 25 cardridge slot
– pin 20 on CS12 i.e. pin 3 cardridge slot

Step 16 : Testing
Now it’s time to test the upgraded NMS8245, to see if it’s a MSX2+
Check if MSX Basic 3.0 is present:

I also use Quasar’s Megareport to do an extra check.
Notice it says 2 PLUS / Modified MSX-2
MSX-Video : V9958

Also when booting, notice the new MSX2plus Boot Logo:

Good Luck upgrading your NMS8245