There is no “before” or “after”. They work in parallel (category and tags). It doesn’t matter if a user selects tags before or after entering some specific categories.
If they enter the categories they want, and only select the tags after, in the end, it will be the same. Categories won’t just be filtered by tags while they enter them, but it probably isn’t very important.
If you mean just the UX, the display of the boxes to select them, I guess that must be possible to switch pretty easily: Tags to the left of categories selection, and not to the right. Though a theme or CSS.