I purchased madden 15 on xbox one and was having trouble getting my xbox live account linked. The Origin client was saying my xbox live account had already been linked to another account. Apparently I created another origin account in the past on xbox 360.
I contacted origin online support and told them I would like to link my origin xbox account (hotmail) to my origin client account (gmail) and provided my xbox live id.
The game advisor sent a verification code to both email addresses. After providing the code, he informed me that any in game purchases on my old xbox origin account may be lost. No big deal to me, those games are old anyway and I'll probably never play them again.
This in no way affected any data on my pc account, all my pc games remained in tact and I am now able to log in to Madden 15 on xbox one. All future games should now be linked to the correct account. I haven't turned on my 360 to find out what data was lost. I guess it could have lost card packs or other in game purchases, but whatever. The main concern was getting things working properly going forward, and it seems like that's what happened.
Hopefully this helps!
