Cases we should revalidate our manufacturing process

Validation
1

in what cases process validation should be revised and another protocol will be written
(solid form)
should we revalidate if we change lactose fast flo to lactose NF

2

any major modification occurred go for revalidation

3

like what?
if i add another step like another sifting after blending with a higher mesh
do i need revalidation for the next three batches?