You kinda can - when you purchase RIs, you're really just purchasing a chunk of capacity ("Units" in EC2 parlance). You can reallocate your units into different RIs, so if you bought a c3.2xlarge and want to upgrade it to a 4x large, you'd just buy a second 2xlarge and then recombine your two 2xlarges into a 4xlarge. Or you could split a 32-unit 4xlarge into 4 8-unit 1x larges.
ie, 2x c3.xlarge (http://i.imgur.com/hPl2WNJ.png) recombine into 1x c3.2xlarge (http://i.imgur.com/KhthjYo.png)
Granted, you can't jump from a medium to a large; it only works within the same product class. But it does provide upgrade paths. It is bass-ackwards, though. And the fact that they sell "instances" rather than "units" but don't let you bind RIs to actual instances is confusing as all get-out.