Do Currency Rate Between PO Header & All Distribution Must Be Same?

  Problem Description/Question:

- PO header currency is JPY > Line currency is JPY with distribution rate = 0.0123 (e.g.)

- Click (B)Currency for change currency to USD > PO header currency has been changed, PO distribution rate NOT be synchronized

- Tools > convert currency > distribution currency has been synchronized.

QUESTION:

Do Currency Rate Between PO Header & Distribution Must Be Same?

If not perform tools > convert currency, is it ok?

Would like to confirm whether PO header currency rate must be same with PO distribution level?

If it is intended functionality to have all PO lines having same rate as PO Header?