input <- "g.chr17:7577121G>A"
input <- sub("^([^.]+\.)([^:]+:)", "\2\1", input)
input

[1] "chr17:g.7577121G>A"

^                from the beginning of the input
    ([^.]+\.)   match and capture any non dot characters up to and including
                 the first dot
    ([^:]+:)     then match and capture any non colon characters up to and
                 including the first colon

sub("(^.{2})(.*:)(.*)", "\2\1\3", x)
#[1] "chr17:g.7577121G>A" "chr3:g.52712586T>C"

x <- c("g.chr17:7577121G>A", "g.chr3:52712586T>C")

v1 <- strsplit(input, "[.:]")[[1]]
paste0(v1[2], ":", v1[1], ".", v1[3])
#[1] "chr17:g.7577121G>A"

input <- "g.chr17:7577121G>A"