For example, Rename FROM "/programfilesx86\AutoCAD 2008\cass.dll" TO cass.dll.bak, If the cass.dll have rename to cass.dll.bak, run this command the second time will cause a prompt, Need a "Quiet" to ignore this.
You could make your button check if the source and/or dest currently exist.
Would you like give an example for this? I can't get any clue in manual.
What do you want to do exactly? (The whole button.)
There are two secondary development software for AutoCAD, one of which will install a file called cass.dll in the AutoCAD installation directory. When this file exists, another software call AutoCAD starts and an error occurs, so it needs to be started. Before judging whether this cass.dll file exists.
something like
Rename FROM "/programfilesx86\AutoCAD 2008\cass.dll" TO cass.dll.bak
/programfilesx86\AutoCAD 2008\acad.exe /P CAD
and
Rename FROM "/programfilesx86\AutoCAD 2008\cass.dll.bak" TO cass.dll
/programfilesx86\AutoCAD 2008\acad.exe /P CASS92 /nologo acad.dwt
@ifexists should be able to take care of that:
https://www.gpsoft.com.au/help/opus12/index.html#!Documents/Command_modifier_reference.htm
Works great, I've forgotten this. 
In the next update we'll add a WHENEXISTS argument which will let you control what happens if the new filename already exists.