Here is one idea..

If you find a defect, you create a new defect type work item for that issue and once it is resolved you apply it on the latest release. If you have a need to apply the same fix on a current version of the product, there is a test fix request work item, where that can be made. Given that the approval is made, the staff maintaining the existing code base would create additional maintenance item or task work items to back port the necessary change sets to the earlier version. Once the fix is prepared the test fix request status is changed to something appropriate like delivered. Then at a regular interval all of the fixes delivered in test fix requests can be rolled up and applied together.

The iFix system works something like this.