Conditional statement that modifies two variables

I have the following data frame:

dat<-data.frame(site=c(rep("A", 3), rep("B", 3)), 
               landuse= rep(c("urban", "dev", "undev"),2), 
               percent= c(30,30,40, 50, 30, 20)) 

For each site, I want to filter for rows where the percent is greater than or equal to 50, but for sites where all landuse categories do not meet the minimum criteria, the landuse entry is changed to "mixed" and the percent is changed to 100.

The result data frame would look like this:

 result<- data.frame(site= c("A", "B"), landuse=c("mixed", "urban"), percent= c(100, 50))