I had similar situation... EU - US - EU in Business. I flew the EU-US and want to change the return. The credit I have is $1499 but the Exec Plat desk told me that in order to use the return, the leg would be treated as a one way and I could use the $1499 credit towards $14,000 one way ticket in business. The credit can only be used for International between either the original city and anywhere in EU or to the arrival city from anywhere in the US.

So basically for me, the credit is worthless. I ended up purchasing another r/t in business for $3200 and hopefully I'll be able to use the return.