You cannot change the type of an existing field, however you can move its value to a newly created LongText field and then delete the old field.
The way to do this is by first creating a new field of type Long text. After that you can run the following code which will set the data of the long text field to be the same as the data of the short text field.
var providerName = String.Empty;
DynamicModuleManager dynamicModuleManager = DynamicModuleManager.GetManager(providerName);
// Resolve dynamic content item types (plural name in yellow, singular name in red)
Type magazineType = TypeResolutionService.ResolveType(
var allMagazines = dynamicModuleManager.GetDataItems(magazineType).Where(i => i.Status == ContentLifecycleStatus.Master);
In order to use this code the following namespaces must be included:
The last step is to remove the Short text field from the module.
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