Need an explanation - what is the different between - SFSN and CIDP? can SFSN turned into CIDP? IF yes - after how long?

I'm going to see a new neurologist tomorrow. Up until now I was DX (after 2.5 years) with idiopatic SFSN. Lately - I have new burning pains in my legs, especially - in the right one. I'm afraid of new DX.
What kind of tests do I have to have in order to DX as CIDP?

Please, need your help. Be nice, don't frighten me...