As noted on the poweroff state of the "Fast-Resize" page:
"Fast Resize: Will allocate the upgraded RAM and Cores for your virtual server without making any disk changes. 1 Minute downtime."
By skipping the disk resize, we are able to shorten the time required to resize server RAM and CPU.
To change the disk size use a snapshot/re-deploy method instead:
1) take a snapshot
2) Optional but recommended: verify the snapshot (create/boot/destroy a test droplet of the desired size)
3) destroy your old droplet (this releases the IP)
4) create a droplet of the desired size using the snapshot as a starting image. This will get the IP released above.