The subtype field cannot be updated in our mass attribute update screen because it controls domains throughout much of our data model. If we allowed you to change the subtype, you could enter into an situation with domain-driven values on each of those subtypes are incorrect. If you must mass update a subtype field, you can use the core esri tools, but remember, this can cause data quality issues.
For example, if you change a 1 phase overhead primary line to 2 phase, the domain driving the phase field changes (from A, B, C to AB, AC, BC) and the phase values on those features will no longer be valid. This can cause problems when tracing.
Thanks Neil, I thought that was the reason but I needed to ask to confirm it.