Adding additional fields to the content type will not break
any of your existing workflows. New instances will start against the new
versions and running instances will continue to execute on the previous version
of the workflow and therefor will keep existing production data intact. The
only thing to consider is removing or renaming (K2 uses the Title of the column)
fields in the content type that your current workflow rely on can cause issues.
As long as you keep this in mind K2 process versioning will keep things going
without issues.

Thanks for responding,

Actually my problem is more about the deployment : I have to add about 30 new fields (site columns) to my content type. The point here is to deploy those new fields to other farms in a minimum amount of time and effort (maybe some tool which would extract the Content type and merge it), but without screwing with my K2 workflow (if the workflow relies on the fields GUID to do his updates, it will fail on the testing environnement because they will not be the same as in the development environnement.

The question is : Does K2 rely on the SharePoint fields' GUID to update them, or does it rely on their internal name ?

Regards