How were you renaming them? Manually renaming and using scripts renaming them works for me.

mIRC1:
Code
alias dde_test {
  var %x $rand(1,2000)
  ddeserver on $version
  ddeserver on testing
  ddeserver on %x
  dde -e foobar command /ddeserver on mIRC_2
  dde -e mIRC_2 command /ddeserver on mIRC_3
  dde -e mIRC_3 command /dde -e %x command /echo -ag Works for me. 
  dde -e mIRC_3 command /ddeserver on foobar
  dde -e foobar command /dde -e %x command /echo -ag mIRC2 renamed into foobar.
}


mIRC2:
Code
on *:start:{ ddeserver on foobar }



Results in mIRC1:

Code
* DDE Server is on (7.63)
* DDE Server is on (testing)
* DDE Server is on (191)
Works for me.
mIRC2 renamed into foobar.