Keysight Technologies 3458A Guide D'utilisation page 399

BASIC Language for the 3458A 7
if a subprogram nesting error exists when the SUBEND command is executed
(e.g., if one of the called subprograms does not exist in 3458AA memory).
If you create or download a subprogram using a subprogram name which already
exists in 3458AA memory, the new subprogram overwrites the previous
subprogram.
Keysight 3458A User's Guide 399