
If you remember your ID and the email for the ID and password then yes you can link it again.I have a o3ds, I have a network id already linked to current current 3ds.
My question is can I link back the same ID if I format my system?

Then go for it, formatting the 3ds and linking it to the same 3ds is just fine.It will ask you to connect online to format the 3DS this will not update your 3DS so dont worryI remember the id and email and I am trying to relink to the same 3DS.

I would downgrade after linking your NNID as I don't want to give you false information as I've downgraded are re-linked myself. Make sure to make a NAND backup before downgrading though.Thanks!
Does this works too if I downgrade my firmware and reformat on the same 3ds but different version of firmware? 8.1 -> 4.5