In ESDS, fixed-length RRDS, and fixed-length RRDS, the record length is fixed- will not change even update the record. However, the length of the record will change when you re-write to KSDS VB (Variable-length) records.
In CICS, a file with fixed-length records, when you re-write the length must be equal to the original length.
Interview Question – How to do in CICS when re-writing the Records
However, when writing to a file with fixed-length records, you need not specify the LENGTH option. If you do, its value is checked against the defined value and you get a LENGERR condition if the values do not match.
For variable-length records, you must specify the LENGTH option with both the READ UPDATE and the REWRITE commands. The length must not be greater than the maximum defined to VSAM.
So, there is a difference between Fixed length and Variable-length records during record UPDATE in CICS.