Hi Mandy. You can initialize MM Periods if ML is active, so its a must you deactivete ML.
If you dont post GM in your system, you can deactivate ML without fear. When you deactivate ML, all the history in ML tables of your plant is rest, so you can any longer check the info in CM3N.
In order to deactivate ML, run program SAPRCKMJX. System create a log. It works well i never have any problem deactivating ML, my recomendation is test in a dev system
For more details check note Note 108374 - Canceling material ledger production startup and the referenced notes.
PS. doit at your own risk, if you are not sure post OSS to to SAP.
Arturo.