I am currently using the Enterprise edition. If I move to a lower edition (Standard or Professional), will I lose my existing data?

I am currently using the Enterprise edition. If I move to a lower edition (Standard or Professional), will I lose my existing data?

No. Your data will be stored securely in your CRM account. When you move from a higher to lower edition, you will not be able to access certain features that are only available in higher editions.
The existing data will be retained in your account regardless of the edition limits. For example: Iyou added 20 custom fields to the Leads module while you were using the Enterprise edition, then move to the Standard edition, you will be able to view only 10 of the 20 fields. However, the data in those hidden fields will be retained in the records. Any customizations like workflows, blueprint, etc. will be marked as inactive or hidden. Only the customizations that are available within the limits of your new edition will be kept active.