AND(
$Profile.Name <> 'Custom',
$Profile.Name <> 'System Administrator',
CONTAINS(RecordType.Name , "Custom"),
NOT(ISNEW()),
OR(
NOT(ISPICKVAL(PRIORVALUE(Status__c), "Status A"))
,
AND(
ISCHANGED(Status__c),
ISPICKVAL(PRIORVALUE(Status__c), "Status A"),
NOT(ISPICKVAL(Status__c, "Status B"))
)
)
)