If you need to copy paste some text in document together with tracked changes to a new document, just follow this simple steps:
1. Turn off track changes in both documents
2. Copy text from the source document and paste to the destination document. You should be able to see all the tracked changes/comments etc in the new document now.
3. Turn on the tracking again, if you want to go to the old state.
How does Track Changes in Microsoft Word work? | ShaunaKelly.com
An excellent article of tracking changes in documents