If you open up your IDM driver, click Synchronize.  From here you can leave at Examine all objects which does a fresh push all, or Use a starting date to pick how far back you want to grab changes from.  The starting date option is helpful if for example your system was down for a week or two and you just want to bring those changes back in.

Here's screenshots of an example system

Cool! I have never used that and did not know it exists!  Thanks!

Sorry it doesn't work for objects created in AD before I start the driver ;(

What you want to do is Migrate from Application. The migrate should update both sides to match the data, based on the merge authority. Which I concede that Merge Authority can be confusing.

You build a Query doc to define the users who should be considered.  If you can define a better query, worst case you can pull in everybody. 

You can use this https://www.netiq.com/documentation/identity-manager-49/driver_admin/data/b1j11hn.html

Use the -migrateapp option where you specify query just like you would in the event in the driver.

Examples can be also found in driver trace where driver queries for any destination data.

Command example:

XML file example:

<nds dtdversion="4.0" ndsversion="8.x">
  <source>
    <product edition="Standard" version="4.0.1.1">DirXML</product>
    <contact>Novell, Inc.</contact>
  </source>
  <input>
    <query class-name="User" scope="subtree" dest-dn="xxxxxxx">
      <search-class class-name="User"/>
      <search-attr attr-name="CN">
        <value>*username*</value>
      </search-attr>
    </query>
  </input>
</nds>