OK

• It's OK to move a task from one category to another, but to keep the responses make sure the task do not change its ID.
  
• It's OK to restructure the task-list setup (add/change/remove categories). Keep the IDs on tasks intact.
  
• It's OK to remove questions without responses.
  

OK-ISH

• If you remove a task with responses, the responses will be lost and the overall data will loose consistency.
  
• It could be OK to add tasks, but be aware that this might impact the consistency if the task "eat" from other tasks. Hopefully the new task will eat from the "Other"-category.
  

Not OK

• It is NOT OK to rename a task text to a completely different task when the task has data. If you change Book ticket to Contact support you are claiming that everyone that attempted to Book a ticket actually attempted to Contact support. Instead, create a new task for Contact support and leave the old task there to keep data consistent.
  

Make annotations
Take a note when you did the change (and what you did), so you can take the change into consideration when you compare analysis in different time intervals.