Inserting random letters at random locations within a string

I am trying to make a little script to demonstrate how DNA sequences can evolve using a sentence as an example. I would like to repeatedly replace or insert letters or words into a string in R. I would like this to happen repeatedly so one can watch the string change over time. Finally I would like there to be a greater probability of letters changing than words changing.

So far I have defined a string and created lists of both letters and words and sample randomly from both these lists.

However I do not know how to then modify the text with a set probability. For example how do I make it so there is a 50% chance of a letter in the text being replaced with a letter from my letter list and if this happens, it should occur at a random location in the text?

I also want this process to occur X times so I can show the text changing over time. Any help or suggestions are greatly appreciated. My current incomplete code is below

#First I define the string
text <- c("This sentence is changing")


#Then make a vector of words from the string
word_list <- strsplit(text, " ")
word_list <- unlist(word_list)


#Also make a vector of letters from the string
letters_and_gaps <- substring(text, seq(1, nchar(text), 1), seq(1, nchar(text), 1))
letters_and_gaps <- unlist(letters_and_gaps)

#Now for probability 1 in 2 or it occuring, select a random character from letters_and_gaps:
sample(letters_and_gaps, 1)
#Then choose a random character in text and replace it with this randomly sampled character:

#Now with probability 1 in 10 or it occuring, select a random word from word_list
sample(letters_and_gaps, 1)
#Then choose a random word in text and replace it with this randomly sampled word:

#Then print the updated text:
text 

#Iteratively repeat this process X times

My goal is to ultimately put this in a Shiny app where one can select the probability of different events occuring (letter vs word replacement) and then watch how this influence how the text evolves.