Import question regarding custom fields

I am importing a list in CSV format.  Currently the fields are First name, Last name, Email.

Let's say however, in my Mailbuild account I have the following custom fields:  First name, Last name, Email, Phone, City.

If in a week I want to reimport this "master list" having filled in the Phone and City fields, will it put the new information with the appropriate contacts? 

Thanks,

D

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