Yes, if you modify an active Competition, the changes will be reflected once you save your campaign.

Here are some scenarios:

Add a New Entry Method

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

Remove an Existing Entry Method

If you removed an existing entry method, new entrants will not be able to complete the action any more. However, existing entries will remain in your Entries/Actions tab.

Change the Campaign End Date

If you changed the campaign end date to today, then the campaign will end and the widget will stop collecting entries.

Similarly, you can change the end date to a future date to extend your Competition campaign.