I guess you’re referring to moving the existing topics to new categories?
If so - yeah, it was a lot 
I think at that point, we had 45K+ topics across all the categories. Luckily, almost all of them were in the “Discussion> Ask a question” category, which we moved to be a main category, renamed as “Ask the community” and then moved the other categories’ topics into it.
Overall, it took ~1 full day of work to make the changes, and then obviously planning in advance.
For example, mapping all of the old pinned topics, and the permalinks needed to redirect to the new pinned topics for each new category.
We also took the time to communicate in advance to users - Studio Community Updates: What’s New - Be in the know - Community Support Forum | Wix Studio - explaining the decisions and the timeframe, which was super important to ensure regular users wouldn’t be overwhelmed by the change.