Mutate a subset of a variable based on a condition

Question

I have the following data…

have_df <- tibble(
    year = c(2015, 2016, 2017, 2018, 2019, 2015, 2016, 2017, 2018, 2019),
    da_assist = c(0, 0, 0, 1, 1, 0,