Paying annually and upgrading...

Hi!

I'm thinking of getting a Linode 64. I'd like to pay annually, mostly because it's more convenient to me.

64Mb however, may end up being a bit tight, so I'd like to know how annual payments play with either upgrading to a larger plan or getting extra RAM.

Am I restricted to upgrading at the end of the year? Or, can I pay the diference regarding the rest of the year monthly or (even better) in another single payment?

One other thing, what are your policies regarding downgrades?

Thanks in advance!