ModifiedOn field data migration

Hello All,

We recently did migration from other system to MS CRM 2016.  Now all the activities are displaying the modified date as the date when we migrated the data.  I have tried creating a plugin based on the information which was provided here

http://missdynamicscrm.blogspot.com/2014/12/update-field-just-before-activity-closed-CRM-sdk-csharp.html

https://community.dynamics.com/crm/b/crmmusings/archive/2013/05/13/crm-2011-migrating-notes-while-maintaining-history-information

If anybody can provide me guidance on this to reset the modify date to the original that would be great.