The following error is reported by the repository: Cannot modify application. It's not allowed to do

Dear Experts,

In the Imaging metadata we have declared a metadata as  type Memo . and also there is another metadata field which is of type long text (but it has picklist values in imaging) .

Now when I am trying to add more values to picklist and then when submitting the application I am getting the following error :

The following error is reported by the repository: Cannot modify application. It's not allowed to downgrade field type from Memo to BigText

Let me know how to add another value to picklist ?

Many thanks for your help in advance