Do you need to re-validate the same re-crystallization step for different reasons for reprocessing? For physical contamination? For out-of-spec particle size? For out-of-spec LOD?

If you’re going to change the process, there’s no question; if you have to change what you do,
you need to validate it. But, if you’re not going to change it, and you’re just going to go right back
in, exactly the way you were doing it the first time through, you may need comprehensive
additional testing, and analysis, and monitoring. But, you do not usually have to validate it.
The process is designed to do something. If you have not already demonstrated that the process
will do something, like reduce a given impurity, then if you’re going to repeat a recrystallization to
remove an impurity, to change a particle size, to do something that you have not already
demonstrated that that process does, that is something that has to be validated possibly as a
rework.