"As others have stated, the CLM solution does not yet have true support for versioned linking. IMO, the best way to achieve what you desire is to keep separate collections and copies of the requirements for each version you need "frozen". You can use attributes and links on the requirements to relate the copies to each other so that you can identify and trace their lineage. Although copies are not ideal, they allow the type of linking and evolution that you desire. "

1) I would keep everything in one document.
2) So I would have a main document describing the requirement.
3) Then I would create sub-items stating what is being considered/implemented in a particular phase/release
4) and what is implemented in another release.
5) These as suggested should be children of the main document as a separate artifact so that it can be individually linked.
6) So each particular implementation of the requirement can be linked to it's own workitem/tasks for implementation and testcases.
7) Interested parties of the requirement can then look at the document and understand what is being implement in all the releases and how changes will impact the different implementations

My approach would be to have a parent/child relationship. The parent is the requirement and the first child is the first version. That way a test case can be linked to the parent and first version while second version of the requirement is being developed for a future release. This will allowing for parallel working.