Subscriber Import wizard provides two options to import: Normal and SmartSubscribe.
The original import method is known as the Normal subscribe method. Normal subscribe is slower than SmartSubscribe, but may be useful when you are importing smaller files. In addition, Normal Subscribe will not clear data if the the field in your file is blank
SmartSubscribe is enabled by default in the Import Subscriber wizard and should be used for all imports unless you have made a business decision to use the slower Normal import method so as to prevent data from being cleared.
To Import using Normal Subscribe, follow the steps in Import Subscribers, and be sure to deselect the 'Use SmartSubscribe' option on the first page of the Import wizard.