the sql server ntfs permissions are secured by the use of either local groups in ealier versions of windows or by virtual server accounts for Windows 2008 on. As the group or virtual acct map to the NTFS ACLs changing the sql server service acct is not affected, the new service acct is mapped to the group or virtual service acct.
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"Ya can't make an omelette without breaking just a few eggs" 😉