It is not possible to revert the changes to the previous version for the Module after the ReqIF import.

As the best practice,

For ReqIF imports into DNG :

1)It is recommended that a new changeset be created for each separate import. 

2)It is recommended that such changesets either be delivered or discarded before importing further data from the same source.

3)We can see the history of the artifacts for the Module and revert the changes manually.

4)Also if you have taken the baseline of the stream before the ReqIF import, you can create the stream from the baseline for viewing the previous versions of the module.

The Main advantage of creating the changeset is that you can discard the changes if it is not required to deliver the changes to the stream and you can revert back to the older changes in the stream(previous version).

https://www.ibm.com/docs/en/elm/7.0.2?topic=configurations-managing-changes-artifacts

There is already an RFE raised for the same. Kindly review and upvote for the same.

https://www.ibm.com/developerworks/rfe/execute?use_case=viewRfe&CR_ID=128488