The user's guide states that if one doesn't set a BOUNDS the default is the entire record:
Changes the contents of a record to all uppercase within the boundaries set by the BOUNDS command. If no BOUNDS command was issued, the default is the entire record.
Note: In MF mode, the fields within the boundaries must be either all EBCDIC or all UTF-16.
If one puts in a bounds value first, i.e. BOUNDS 1 50, then puts a UC in a line that has lower case data in the packed fields and they will be untouched.