There might be more elegant solutions but this is just a quick example how it could be achieved. I've subdivided my plane fewer times, but the principle works with your high resolution as well.
I'm looking top down on the plane, so I wanted to change the rows from Y to -Y. For that I used a Separate XYZ node and checked the Y value if it's Less Than a certain value, which is animated from from a positive value down to a negative value. The result "no" (0) or "yes" is then plugged into the Seed input of a Random Value node, basically making the setup switch the the seed from 0 to 1 going down the Y axis.
Of course this setup can be tweaked and manipulated to switch between other seeds etc. and maybe there are better solutions. But here's how it looks as is now:
I guess there are other ways to switch the output text better and the other thing is, if you would use Mesh Primitives > Grid inside Geometry Nodes there might also be a way to select via IDs and so on. As I said, this is just to give you an idea what's possible. Here's the node setup:
