By design, it isn't possible to change the version number of a KM in a policy.
The solution would be to rebuild the policy.
Note that in later versions of the product (e.g. TrueSight 11), there is the concept of copying a policy. It is still restrictive of making changes to the monitor (e.g. version), but it would same you time nonetheless.
You may want to vote up these two ideas:
Note: some KMs will allow you to export the policy, edit the version, and import it back into BPPM. *But not all KMs.*
There isn't any list of KMs which allow this (and even so, we've seen success with some versions of a given KM, and failure in other versions of the same KM). The recommendation is therefore to rebuild the policy. However, if you want to try the re-import method, note that all steps will appear to be successful; the only way to confirm success is to check the configuration of the monitor in the policy to see if the details are present.