How to I restore my old default keychain?
Ok I can answer by myself. Doing the above, keychain will in fact store a copy of the old 'defaults' (all the previous... Don't why they're called 'defaults'). The copy (at least in Sierra) is located in 

~/Library/Keychains/login_renamed_1.keychain-db


You can have back all your data by opening Keychain and drag/drop that file on the left column, on the top area. Once done, you'll be able to find all your old data below (Password, keys, etc...)

The apps will find them again, eventually asking you the password (the login one) to open the backed-up ones.