Is it possible to change the ItemID of a Portal Item (e.g. a webmap) to a specific value, or to set a specific ItemID when creating a new item? I don't see any API supporting that so I assume it isn't possible...is my assumption correct?
You are correct, it is not possible to specify an itemID or modify an existing one, although I do see some potential value in doing so for certain use cases.
If it does not already exist, I would recommend posting an idea for this here on GeoNet.