If for some reason it is preferable to have an actual switch between two servers you can always code the jobs with a variable in the agent field. You can then put the actual agent name in the variable and modify it at will or with a CLI command. One caveat to observe is the fact that the variable will be resolved at the time the application generates, not at the point the job runs.
Example: %VAR('server_agent','FTP') where 'server_agent' is the name of the agent you want and 'FTP' is the Global Variable context