As you already know, your gender will indeed be changed to what the gender of the account is. You will, however, retain all of your items and zeny that were located on that character and inside their cart! Keep in mind that items that are in your Kafra will not be moved.
As for slots, you will need to select a slot that is not filled. You need to be specific on where you want it to be moved. If you select that you want it on the 2nd slot, but there is already a character on the 2nd slot, your spend credit will most likely be closed and you will need to re-submit a new spend credit with an empty slot.
