Hello Kaushik,
As Lakshmipathi Sir has rightfully pointed out, you can not make changes to document currency field at the sales document header level if you have any subsequent documents created, and in your case we see that there are delivery, bunch of invoices and down payment request doc etc generated.
If you want to make changes then you should cancel all the subsequent documents and then make changes, but you better try it in test client before carrying it out in production.Image may be NSFW.
Clik here to view.
Regards,
Naveen K.
