Decide if an agent may rewrite a serial parser

Your team maintains C++ code that frames and parses a binary protocol over a serial link to field devices. Messages are length-prefixed, carry a CRC (a checksum that detects corrupted bytes) and have strict byte layouts. The parser is ugly, but it has run on 20,000 devices for 6 years. A colleague wants an agent to rewrite it "to be cleaner".

Do you let the agent rewrite it? Decide what it may change and what must stay as it is.