I have just created an account to say this: CHANGING THE ID OF THE PLAN IS INSANE AND CLEARLY A BAD-DEVELOPER CHOICE.
I bet there is a piece of code in Linode codebase that has the plan resources mapped in a table, with a list of ID attached to the various sizes. The genius developer decided that it makes sense to have the "growing number ID" match the "growth in resources" assigned to the plan. In other words, the bigger ID should always mean MORE resources.
This leads them to alter IDs if they have to introduce plan-sizes that are small.
Imagine if AWS did this: you would see riots in the streets!