There are a few different methods for moving ideas between forums and categories in UserVoice.
Bulk moving ideas
1) In the idea grid, use the checkboxes to select multiple ideas you wish to move.
2) Click on "More actions" and choose "Move ideas."
3) In the pop-up, you will see options to select a for the category and/or forum to move the ideas into.
Moving individual ideas
While you can use the above bulk method to move a single idea, there is another way to move a single idea between categories or forums.
1) On an idea, click the three dots and choose "Edit."
2) From the pop-up, you will see corresponding drop-down options to change the forum or category of the idea. Change these, and click "Save changes" to move the idea.