Random Numbers animating to letters After Effects Animation Workaround

You're in luck : )

Here's an alternative approach - but it has a few limitations:

1. Requires a mono-spaced font.
2. Single line of text - doesn't work with line breaks or blocks of text.
3. Designed to work with opacity. (Range selector > Advanced > Smoothness can be any value.)
4. It's a two layer solution.

If that meets your needs - it may be a little faster in use.

On the positive side:
Any style or text changes made to the text layer will automatically update the number layer : )

Here's what to do:

1. Set your text and animate this with a text animator as normal.
2. Duplicate the layer.
3. On the lower layer:

          Set: Animator 1 > Range selector > Advanced > Mode : subtract

          Paste this expression into the Text > Source text property

posterizeTime(5); // for fps of random number change
txtLen = thisComp.layer(index-1).text.sourceText.length;
txtAll = "";
for (var i = 0; i < txtLen; i++ ) {
txt = Math.floor(random(0,9))
txtAll += txt
} 
thisComp.layer(index-1).text.sourceText.style.setText(txtAll)

Adjust the posterizeTime property to taste.

Optional: you may want to parent the lower layer to the main layer so that any changes to to the parent layer's position, scale and rotation are matched by the number layer.