There is no "override" function in VersionOne as the tool is meant to be used as the primary means of team collaboration. However, if this is something that a number of VersionOne users would like to see, add the suggestion here:
https://ideaspace.versionone.com/default
Rather than copy and paste the changes (via the Backlog grid view), you would probably just want to create a new space and load the 2600 in there and work in that new space. Close the old space off as it sounds like the 2600 backlog items is the new "baseline" from which your teams will work off.
Out of curiosity, do you honestly think your project will complete the 2600 backlog items? It sounds like you've got a waterfall project and with 2600 as the new baseline ... geez, it doesn't sound like it is going to be manageable ... but, all the best with it!