You can, however, capture an AMI image of it and boot up a new one in a larger instance size. If you're using ElasticIP you can associate the address with the new instance, or you can change a CNAME in Route53 as long as you have a nice low TTL (like, 60 seconds).
It's not quite as seamless as rebooting to a new size, but has the advantage that you can boot it up while the smaller instance is still running, and then you can just flip the switch. In my experience, an ElasticIP association can take a few minutes,but once it's off the old one and onto the new one, there's no risk of connections to the old one continuing. The Route53 trick might work a little better depending on what you're needing to do. I think you can set the Route53 TTL as low as 5 seconds.
Also be aware that the process of capturing an AMI takes about half an hour and reboots the instance, so it's not something you'll want to do on the fly, but rather ahead of time.