Upgrading takes place in a couple of seconds, really. At the most a few minutes and you're done.
Now, downgrading (at least for now.) is the tricky part: You can't upgrade the storage of a VPS you have previously created. For instance, if you first created the 1GB droplet (30GB SSD) and you happen to upgrade to the 48GB package (480GB SSD) you would still have the 30GB SSD (using the fast-resize option which takes only a couple seconds and preserves all the data and configuration you have previously done.) and BTW: you'd still be paying the 480GB while having only 30GB available.
That happens due to the way virtual disks (and disks in general.) are handled. You can't just override the "physical" block counter of a disk and say "hey, there is more space now, grow it!". So the workaround is to take a snapshot of your lower-spec droplet and create a brand new one with the 48GB package using that snapshopt as a starting point. That way you'll enjoy all the benefits of the new plan and won't be losing any specs or money.
The downside to this is that you are likely (not certainly, but there is a considerable chance.) of getting a NEW IP address. So that might mean a lot, or nothing, depending on your use of it. Depending on how big you intend to go, there's always the option you might add redundancies and all sort of workarounds so that your service won't be completely down or would just experience a very insignificant downtime. It's all up to your planning and imagination on that part.
Just to cover the last point: If you, however, choose a basic plan (let's say, 1GB.) you may upgrade to 48GB and downgrade back to 1GB just as quick while preserving EVERY aspect of your droplet. No hassle, no problems, no IP changing. Then that "boost" time will be charged hourly. :)
Hope to have helped!