Here's the situation:

I have bought a ticket in Y for 591 EUR. Fully flexible.

I later found out that there is a cheaper fare in Business (class D) for 518 EUR. I therefore want to change to the lower fare.

The travel agent tells me that he cannot modify the ticket and refund the difference. Instead he has to cancel the original ticket and re-issue a new one (good for him, he can charge the ticket issuance fee a second time). If he only modified the ticket to the higher class but lower fare I would not receive a refund of the difference.

Is that true?