WHO Good manufacturing Guidelines on Validation mentioned in its guideline documents that when to do Requalification of equipment system utilities.
Based on the identified risks and their mitigation strategies based on identified needs to be considered.
Factors like frequency of use or period of use of equipment, breakdown, and based on results of operations, critical natue of equipment, observation during preventive mentenace repair work, any issues in calibration or verification.
If one simillar looking equipment is replaced with similar Equipment bit must be qualified.
If equipment parts are replaced partly it must be Requalified.
In event if a equipment is in use for longer period of time.
In all above mentioned events equipment is required to be Requalified