Some transformers have dual primary windings, such as the CM-75101A and the CMQEE-3440A.  This allows for the windings (and taps) to be switched for different turns ratios and, hence, impedance ratios.  The switch connections are shown here.  

These switches carry very low signal levels.  Very good quality switches are highly recommended for long term trouble-free operation.  

NOTE:  The CM-75101A with the lead wire version can be specially ordered with center-tapped primary windings.