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You can keep using the same flat masters so long as there's no significant change in your imaging train. Slight focus changes are usually OK. Slewing to a new object is fine. Rotating the camera (and filters together) is often OK unless there's some asymmetry of the image circle. What usually stops flats being useful is when you get new dust spots or a dust spot moves on the cover glass or a filter.
Cheers,
Rick.
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