They should be able to do that themselves (I've seen a lot of projects adding tiers in the middle of campaigns, you can't remove/change existing tiers).
Kickstarter shouldn't be able to add new tiers by themselves, as they have no say what the price/rewards should be. The current tiers are likely (hopefully) carefully planned based on their manufacturing plans and how many watches they can fulfill in the schedule given.
Kickstarter shouldn't be able to add new tiers by themselves, as they have no say what the price/rewards should be. The current tiers are likely (hopefully) carefully planned based on their manufacturing plans and how many watches they can fulfill in the schedule given.