In the next Bloons Tower Defense game, it will be possible to remove some items that you have placed (but not anything else). For example, if you have already placed road spikes and a Nuke Bloon is coming, you can remove the road spikes so that they won't pop it. However, not all items will be able to be removed. Depending on the type of item, if it can be removed, you will either lose it or will be able to replace it whenever you want.