If you have a nested list, I suggest you use rapply() from base R. I invented some data that may, or may not, be similar to your data.
DATA <- list(1:3,list(7:9,c('a','s'),677),7:11)
DATA
#> [[1]]
#> [1] 1 2 3
#>
#> [[2]]
#> [[2]][[1]]
#> [1] 7 8 9
#>
#> [[2]][[2]]
#> [1] "a" "s"
#>
#> [[2]][[3]]
#> [1] 677
#>
#>
#> [[3]]
#> [1] 7 8 9 10 11
rapply(DATA,as.character,how="replace")
#> [[1]]
#> [1] "1" "2" "3"
#>
#> [[2]]
#> [[2]][[1]]
#> [1] "7" "8" "9"
#>
#> [[2]][[2]]
#> [1] "a" "s"
#>
#> [[2]][[3]]
#> [1] "677"
#>
#>
#> [[3]]
#> [1] "7" "8" "9" "10" "11"
Created on 2022-07-30 by the reprex package (v2.0.1)