Bill C-309 is not changing the elements that must be proven in either case.
There is a distinction in terms of what must be shown, obviously, for either offence. The unlawful assembly offence in section 66 is a summary conviction offence; it's a lower-level type of offence. You still have to prove all of the conditions of that offence before one can be convicted of it, including a gathering of three or more persons, etc. But it is possible under that offence for somebody to be convicted, if all of those conditions are present and they are aware of the situation and choose to remain passively acquiescent; there is case law to that effect.
What's important for the committee to recall is that Bill C-309 is not changing the fundamental operation of either of those offences. Rather, it concerns what penalty is to be imposed when a person who has committed one of those offences is also wearing a mask to conceal his or her identity without lawful excuse.