[spoiler=Step 2]
Select the desired merge option; once you get the successful merge dialogue, you're done!
I also suggest to read the EVERYTHING ABOUT THE MERGE for questions you may have and if you should experience any issue while merging.
In case that, after the merge, you should still get the Invalid Password/Username error, please go ahead and change your password from the DASHBOARD, ensuring that it is alphanumerical (characters from a to z and from 0 to 9)
Remember that you are able to play, with your old progress, only on the Silver Server since the server transfers are closed in this moment.
Let me know if you have been successful in solving your issue and if you have any further questions.
Have a wonderful day !
Thank you a lot! everything's working fine now. I've started because my friend told me to but he's actually on the gold server, there is no way the migrate right? haha