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:

[![random animation][1]][1]

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:

[![node setup][2]][2]


  [1]: https://i.sstatic.net/dCuHs.gif
  [2]: https://i.sstatic.net/LvjgP.jpg