Agreed - this is something you definitely wanna consider.
If this is the trigger campaign - you can clone this campaign, update the wait step (new campaign can have a single 6 days wait step and old campaign can have the two wait steps - 2 days and 4 days) , and phase from the old campaign to new campaign (deactivate the existing campaign and activate the new campaign at same time) - people already in the older trigger campaign's wait step would progress through the steps (i.e., 2 + 4 days of wait step followed by rest of the flow steps) as usual, deactivating the trigger campaign would simply stop this campaign from any new acquisitions.
Else if this is a batch campaign, you can update the flow to have the second wait step of 4 days right below the 2 days wait step so that people in already in flow go through a total of 6 days wait step, and when there's no-one in the campaign's flow/wait steps - you can simply update the original wait step from 2 days to 6 days and remove the second wait step - alternatively, in-case of the scheduled batch campaign you can clone existing campaign, update the wait step to 6 days and switch to the new campaign (schedule the new campaign appropriately and stop the campaign recurrence for the existing old campaign), again people already in the wait step of existing campaign would continue to progress through the campaign's flow (2 + 4 days wait step followed by rest of the campaign flow step).
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