It's tedious but you actually want to work on the transactions tab to recategorise everything. If you do it from the trend tab, you will have to reallocate transactions from each category then move to the next, and as things start moving around you will see the same thing twice when you get to the category that you moved it too. If you do it from the all transactions page, you can go through he transactions chronologically and they won't move because they are arranged by date only.
Also, If you see reoccurring expenses, you can search for that expense and apply your change to all of the transactions at once, really cuts down on the time to organize