The only determining factor now taken into account by school is the birth-date of the child.    Your child would need to be 4 by 31st August to join FS2 in most UK curriculum schools (though some are now following the cut-off date for international schools of 31st December in which case he would need to be 4 years and 8 months).  You will need to check with the individual schools to which you are applying on this point.  There is no requirement for any transfer certificate or similar at this age.  You do not mention why you want to make this change and it is therefore not possible for me to comment on whether this is advisable or not.