[R] count value changes in a column

[Justin Haynes]

is there a way to look for value changes in a column?

set.seed(144)
df<-data.frame(state=sample(rep(1:5,200),1000))

any of the five states are acceptable.  however if, for example,
states 4 or 5 follow state 3, i want to overwrite them with 3.
changes from 1 to any value and 2 to any value are acceptable as are
changes from any value to 1 or 2.

By way of an example:

the sequence 1 3 3 5 5 3 2 4 2 1 5 3 3 5

should read   1 3 3 3 3 3 2 4 2 1 5 5 5 5


Thanks for the help!

Justin