Equipment Qualification

At which time we can start requalification of equipment as per ICH guidline?

it’s depend on the particular quipment and which effect the changes in calibration volumes.
as per me for the Reactor,
The major changes done like,
1.Changing ajitator in Reactor,
2.Removal of Teflan pads,
3.Modification of Thermowell,

In My knowledge, I have not seen requalification criteria in ICH guideline, if it is there i would like to know to get updated.

For the Requalification you need to define, Why it is required to be requalified?
in What circumstances your equipment required to be under go Requalification.