If a signal segment has the signal name of an existing net present in
the schematic, Eagle refuses to rename it. You will get "please do this
in the schematic" message.
Now, if this segment was "floating", meaning it was not connected to any
of its target pins, or ending in airwires, why prevent this renaming? I
do support this "error" message when it is connected to one or more
target pins (although sometimes I would like to override it).
For now, I will write this function into my toolbox (ulp).