Change Data Type from String to Long in Realm Android using RealmMigration

final RealmObjectSchema customerSchema = schema.get("Customer");
customerSchema.removeField("creditPeriod")
    .addField("creditPeriod", Long.class);

Above is the code i used for realm migration. I have deleted the already existing field which was previously string and then added the same field changing its data type in migration code and also in Pojo.class

Below I just mentioned a example for migrating the field type from String to int using the example mentioned in the comment by @christian-melchior

public class DataMigration implements RealmMigration {
    @Override
    public void migrate(DynamicRealm realm, long oldVersion, long newVersion) {
        RealmSchema schema = realm.getSchema();
            
        ......
        
        // Modify your check according to your version update.
        if (oldVersion == 1) {
            // Below I am going to change the type of field 'id' from 'String' to 'int'

            // Get Schema of a Particular Class. Change --Testing-- with the respected pojo class name
            RealmObjectSchema schema = schema.get("Testing");

            // Add a new temporary field with the new type which you want to migrate. 'id_tmp' is a temporary integer field.
            schema.addField("id_tmp", int.class);

            // Set the previous value to the new temporary field
            schema.transform(new RealmObjectSchema.Function() {
                @Override
                public void apply(DynamicRealmObject obj) {
                    // Implement the functionality to change the type. Below I just transformed a string type to int type by casting the value. Implement your methodology below.
                    String id = obj.getString("id");

                    obj.setInt("id_tmp", Integer.valueOf(id));
                }
            });

            // Remove the existing field
            schema.removeField("id");

            // Rename the temporary field which hold the transformed value to the field name which you insisted to migrate.
            schema.renameField("id_tmp", "id");
            
            oldVersion++;
        }
        
        ......
        
    }
}

Don't forgot to update the schema version and refer the above migration class instance in the RealmConfiguration instance of realm.