Dont know if this is possible at all. Check yourself-it will pop up when buying. Normal procedure is to first buy new pass to your new wishes, pay, then cancel old one (with some form or whatever), and wait+wait+wait to get refund, less fee of 15€-if possible at all. 1country is hardly ever discounted so there are probably not the more restrictive rules on discount passes.

Here are the instructions for how to exchange a pass. If you look at the invoice for your one country pass there should be information if the pass is exchangeable.

This is a link to the form you need to fill in:

https://eurail.zendesk.com/hc/en-001/requests/new