A feeding unit to feed blanks in a packer machine has a frame supporting a hopper and conveyor. The hopper holds a stack of blanks and has a pick-up opening, through which one blank at a time can be retrieved from the stack. The conveyor moves the blanks along a moving path ending in the hopper. The feeding unit can be adjusted by a format change operation to contain blanks with different formats by changing the feeding unit from a first configuration for containing a first format to a second configuration for containing a second format, different from the first format. A plurality of different and interchangeable hoppers are provided, each associated with a corresponding format of the blanks. Each hopper is fixed to the frame in a removable manner in order to be replaced during a format change operation.