I’m not sure I get your question. Why would you want to change your pass?

Pass is your ticket.

For some trains reservations are mandatory or optional. It’s to fix you a seat on a given train. It is something completely separate and not linked to the pass (only linked with your name).

Should you change passes, your reservations remain valid.

You will probably lose as much in the cost of the PLUS, which will not be refunded, as in rebuying with 10% off and rebuying your PLUS.

I think you mean should you apply for a full refund on your present pass and then buy new ones with the 10% discount. If it saves a money do it but check the cancellation terms and conditions on the email your received. I’ve done this a few times in the past but remember the promotional passes are more restrictive if you want to cancel them.

As Brendan says passes and reservations are completely separate.

Dear All, 

Thank you all for dedicated reply and information to this issue. 

I’ll think again if I should take a risk to replace pass even it is only discount only afterwards.
But I would be appreciated Eurail do pre announcement for sale information ?!?   

The fee for exchanging passes, if you didn't buy PLUS is 15 EUR per pass.

https://www.eurail.com/en/eurail-passes/ordering-info/refund-exchange-policy

Since the process for exchange is you have to buy the new pass (with a new PLUS) and then claim your 100% refund minus your original PLUS fee, you would need to save at least 50 euro to break even. There are few passes with 10% off that save over 50 euro, and then you only save 10 euro per extra 100 euro.

Obviously those without PLUS only need to save the 15 euro exchange fee so the 10% saving would save 10% of everything after 150 euro. e,g, 35 euro on a 500 euro pass.