Charges to upgrade / downgrade?

Nope, and as of recently you can even do it yourself. (Before you had to file a ticket.)

The only thing approaching a "charge" for this is that you don't get any money refunded to you when you downgrade, but of course whatever you've paid remains in your account.

To clarify, when you downgrade, the prorated difference gets stored in your Linode account. It can then be used towards the future purchase of Linode products or renewals.

Or you can ask for a refund, if you decided you didn't need that Linode 14400 for two years after all.