You can easily change the default form used for new items for any folder type except Mail folders.
Although the screenshots are for contacts, the steps are the same for Calendar & Tasks.
- Right click on the folder name and choose Properties.
- Then select the new form from the General tab, When posting to this folder use: field.
- Select the form you want to use.
If the form is published to the current folder, it will be listed in the dropdown; otherwise, you'll need to select Forms... then browse to find it.
Once you do this, when you click the New button or otherwise create a new item of that item type, Outlook will use the form you selected.
Note that you cannot change the default form used for email.
To convert existing items to use the new form, use DocMessageClass or the macro at Set Existing Contacts to use Custom Form using VBA
Note that you can't apply a new form to change the business card on existing contacts. See Changing Microsoft Outlook's Business Card layout for more information.