[SalesForce] Uppercase validation rule

if you want to restrict changes to Account field only if the field already has a value, then you can use something like this

AND(
  NOT(ISBLANK(AccountId)),
  ISCHANGED(AccountId)
)

this will check if the Account field has changed and if it already has a value.

The rule looks ok to me except for the quotes you have used around the values apple & orange..

I took the exact one you had in my dev org and it complained only on the quotes.. change it to single quote like belwo and it should work..

AND(
OR(
ISPICKVAL(Agreement_Type__c, 'apple'),
ISPICKVAL(Agreement_Type__c, 'orange')
),
ISBLANK(Related_Member__c)
)