Yes! You can always modify your campaign setup, even after it has started.

Add a New Redemption Method

If you add a new redemption method, it will appear immediately after you save your campaign. Both new & returning entrants will be able to complete the action.

Remove an Existing Redemption Method

If you removed an existing redemption method, new entrants will not be able to complete the action anymore. However, existing entries will remain in your Actions tab.