It's so easy, if you're not experienced, to screw up the sieve script, so anything that makes you think twice before diving in may be of value to both the user and to Fastmail support.
Personally, I would have no problem with re-authenticating.
If you have strong feelings about it, however, you're best raising the question direct with Fastmail support, as you're unlikely to find them visiting here on this forum.
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