Amazon won't let me buy e.g. a small RI and then pay the difference to upgrade the RI to a medium or large
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.
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.
You cannot just "buy a second 2xlarge" and then recombine them into a 4xlarge. The 2xlarge reservations and pay attention here need to match on their purchase date and hour. I.e they have to be bought within the same clock hour on the same day.
If you reserved a 2xlarge at 2014-12-01T22:59:59Z, you couldn't even combine it with one bought at 2014-12-01T23:00:01Z. Yes Amazon is that picky about it. While you might be able to muscle that through a sales contact, it's not even available to customers on business level support, so anything more than a day apart and you're likely out of luck in any case.
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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.
What you're saying is not really correct.
You cannot just "buy a second 2xlarge" and then recombine them into a 4xlarge. The 2xlarge reservations and pay attention here need to match on their purchase date and hour. I.e they have to be bought within the same clock hour on the same day.
If you reserved a 2xlarge at 2014-12-01T22:59:59Z, you couldn't even combine it with one bought at 2014-12-01T23:00:01Z. Yes Amazon is that picky about it. While you might be able to muscle that through a sales contact, it's not even available to customers on business level support, so anything more than a day apart and you're likely out of luck in any case.
(See: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ri-modify..., particularly bullet point #6)
Ah, I didn't realize this. Wow, that's...restrictive. Thanks for the info.
An important limitation to note is that while you can "just buy a second 2xlarge", you have to do it within the same hour:
"When combining Reserved Instances, the end dates of the Reserved Instances must have the same hour digits in 24-hour format"
Source: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ri-modify...
thanks for the tip, I didnt know thats how it worked.