Opportunity Workflow using ISCHANGED
This is the last variation that I tried:
AND ( ISNULL( Approving_Account_Executive__c ) && ( ISCHANGED( Amount ) || ISCHANGED( StageName ) || ISCHANGED( Probability ) ) )
I've been getting all sorts of messages including ISCHANGED may not be used in this type of formula. The evaluation criteria is set to "When a record is created, or when a record is edited and did not previously meet the rule criteria".
Every time a record is created or edited
so the workflow will be triggered every time any of those fields is changed.
not(isblank(approving_account_executive__c)) && ( ischanged(amount) || ischanged(stagename) || ischanged(probability) )
not(isnew()) to make sure
not(isblank(approving_account_executive__c)) && ( ischanged(amount) || ischanged(stagename) || ischanged(probability) ) && not(isnew())
however, based on my experience, the workflow would not fire off when new opportunity is created, even without not(isnew()).
but you can add it to make sure.