[R] shift by one column given rows in a dataframe

[Massimo Bressan]

by considering the following reproducible example:

v0<-c("a","xxx","c",rep("xxx",2))
v1<-c(1,"b",3,"d","e")
v2<-c(6,2,8,4,5)
v3<-c("xxx",7,"xxx",9,10)

df_start<-data.frame(v0,v1,v2,v3)
df_start

v0<-letters[1:5]
v1<-1:5
v2<-6:10

df_end<-data.frame(v0,v1,v2)
df_end

I need  to shift by one column some given rows in the initial data frame 
called "df_start" so that to get the final structure as in "df_end";
please consider that the value "xxx" in the rows of "df_start" can be 
anything so that I necessarly need to apply by row index position (in my 
reproducible example rows: 2, 3, 5);

I'm really stuck with that problem and I can not conceive any viable 
solution up to now

any hints?

best regards

m