(For R12) In the top banner, you can click "Create version" while you are on the cost source object. Once you have done that, you can change any allocation strategy you want and only months going forward will be impacted, not months behind (if you want it to be all of 2019, make sure to create version in Jan 2019).
For R11, it has been awhile since i used it but i believe you must version the entire model, not just the object.