Here are some of the frequently used data migration options
1. CRM's in-built Import data wizard: This works best in when you have small amount of data and is easy to use. You can read more about it
here and in
implementation guide.
2. Third party tools like
Scribe make it easy to import data from other CRMs with pre-defined mappings etc however, they come at a cost.
3. Custom built solution using CRM Webservices & SSIS packages are used in a complex data transformation requirement or high data loads.
Based on your description, Import Wizard might be something that you could look at.
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