Things to consider:
- What development tools will devs use?
- How long will it take devs to get up to speed on them?
- How does debugging work?
- How do you handle losing breakpoints?
- What does the new build process look like?
- How are you going to handle graceful degradation?
- How does font-end error reporting work?
- How long will it take to make the migration?
- What business metrics are you tracking to acknowledge you made the right user choice? The right technology choice?
- How soon will we see the needle move on those business metrics?
- How will this affect growth in the short term?
- Do your hardcore Flash employees want to be working in HTML5? What will you do if they don't?
Major technology shifts are a lot more complicated than just "code it and see what happens" when you have an established product and team.