There are definitely ways to do this.
In the sentence "How are you?" you can add hooks for the letters you want to change. Like so:
Ho[w]<hook1| are [y]<hook2|ou?
Then you can use the (live:) macro to replace the contents of the hook with various things. Sorry if the below approach seems confusing.
First set up an array of the characters you want replacing the hook:
(set:$hook1_replacement_chars to (a:"c","r","w"))
Then, using a (live:) macro, which repeats on the basis of time you give it, use an index to loop through the array above. Inside the loop, increment the index, and then use a modulus to keep the index giving you 1, 2, 3, replacing the characters in an endless loop:
(set:$index to 1)
(live:1s)[(replace:?hook1)[$hook1_replacement_chars's ($index % 3 + 1)]
(set:$index to ($index + 1)]
You'd have to set up one of these for each hook. Hope it's worth it, haha...