When you first qualify as an FI in EASA-land you are an FI(R), ie "Restricted". This means that you have to work under the supervision of an unrestricted FI, you cannot send students on first solo or first solo nav, you cannot teach applied IF or night.
To remove the basic restriction you must achieve 100 hours instruction and authorise 25 student solo flight.
With further training you can teach night and applied IF.
If you are converting an ICAO rating, a reduced course of 30 hours groundschool and 15 hours flying is required, plus the Assessment of Competence. If you meet the requirements (100/25) you will be issued with an unrestricted certificate.
You would have to complete the relevant EASA training for night or applied IF instruction.
If you would like to PM me I can explain further.
Be aware that the term CFI in UK is used to refer to the Chief Flight Instructor in an ATO.