I agree with Sandy! To add more color, the bi-directional sync solely covers the member status value updates i.e. if the status of the program/campaign members is updated to other legal status values in either of the platforms, the corresponding member status update is synced with the other platform. However, this does not cover the change of the value of the progression step/campaign status values altogether (like you wish to do in your case) - i.e., once the sync is initially done, there are no further updates sent across the SFDC campaign (and vice versa) for updating the campaign status values in case if the progression step values are changed, so you will have to update the progression status manually to both the Marketo program channel as well as in the SFDC campaign. Lastly, if you create a new program with updated channel progression status values, the corresponding SFDC campaign created by the campaign sync will as expected have the new campaign member status values (i.e., the updated status values).
Also, you don't really need to turn the sync off while updating the channel progression steps in Marketo/campaign member statues in SFDC, as anyway, those particular updates won't sync bi-directionally between both platforms (as stated above).
Manually updating campaign member statuses in SFDC could be quite a heavy lift if you already have a lot of synced campaigns, so taking that into consideration it'd make a valid case for just retiring the existing channel and creating a new channel with the updated channel progression steps and use it going forward for your programs in Marketo.
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