You can save global formulas locally, so you can make local changes to the formula without affecting other users.
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Open the global formula you want to save locally
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Click the formula’s column header and select Edit
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At the top of the Formula Editor screen, click the formula’s name
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Select Copy as local field
The formula will now operate as a local field - any changes you make to it will only affect you, and if you close the column those changes will be lost.