Yes, you can replace videos anytime, but revisions do count against your upload quota usage. For revisions, you can easily purchase additional quota in 5GB increments to your deliverable.
The reason behind this is that we incur video transcoding costs each time a video is uploaded and processed, even if it's deleted right after upload. Due to this cost, we cannot offer free revisions. This makes our solution very different than storage-only services like Dropbox.