Can ICAO airline designators be reassigned to a new airline?
Yes. As per rules ICAO can reassign the three letter airline designators after a period of 60 days. 


  When a designator is no longer required, ICAO should be informed immediately. Any such designator so released will not be reassigned until a period of at least 60 days has elapsed. 


The designator BKL was used by Baikal Airlines and is currently used by Aircompany Barcol.

Another example is the airline designator WOW, which was used by Air Southwest  and is currently used by WOW air.