I'm not sure why i want them to change! I asked the same question when i posted the problem last month! The general feeling was they should use the perch and would get the idea easily!
I actually don't mind the dirty eggs, it's them sleeping in a dirty nest i don't like! I was having to clean/tidy it every day! Not doing that has been lovely as its been cold and snowy!
The perch is higher than the nests and its not that long, so they are as close together in the nest as on the perch!
Right i won't worry about it! I'll let them do there own thing!