Your question was answered in my IPS/AV/ABOT Immersion video series; the answer is yes you can change them but it requires the use of GUIDBedit, and future SNORT import operations for the same signature will set the three rating criteria back to their default values. You really should be using the newer Custom Threat Indicators feature instead which is much more flexible and easy to work with.
Attend my 60-minute "Be your Own TAC: Part Deux" Presentation
Exclusively at CPX 2025 Las Vegas Tuesday Feb 25th @ 1:00pm