Is it possible to change the custom field format type after making the custom field? Like changing from Plain Text to Rich Text or vice versa after the custom field has been made, or do you have to make a whole new custom field?
Unfortunately no. The Format of a custom field is the one item about it that is not editable. This is covered in the Custom Fields support doc here:
Note: once you save your custom field, you won't be able to change the format. You can edit anything else about a custom field, just not the type. If you decide you need a different format, you can just create another custom field and choose a different format.
Hope that helps!