Hello @svuxdtbg0tpz,
if you unlink your Nintendo account from your first EA account, you will lose your items, and it can cause other issues as well. There is no way to merge the two accounts in order to take advantage of the cross progression feature without losing a significant amount of things on one account.
This is also said in the article regarding unlinking accounts:
https://help.ea.com/en/help/account/unlink-gaming-accounts-from-ea-account/
Bitte schickt mir keine Direktnachrichten für Support - taggt mich im Forum @EA_Pythia