Validation rule to prevent account name change on opportunity

Question

I am trying to write a validation rule to prevent users from changing Account name on the Opportunity once a value is present in a custom text field on that Opportunity.

If I use ISCHANGED(Account.Name) I get

"Error: The ISCHANGED function cannot reference the Account.Name field."

and when I try with ISCHANGED(Account) I get

" Error: Field Account does not exist. Check spelling."

Can you please help me with the formula?

Answer 1

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.