I've inherited a structure for Tag Manager containers that have each new container in a new account. Can I move the existing containers into one or 2 accounts so that I can group them into logical themes e.g. have one account for ecommerce sites, one account for blog sites, that sort of thing, instead of the current messy structure of every container being in a separate account
You can't move a container from one account to another but you can export a container and then import it into a new account. Take note of the fact though that it will be a new container with new container ID. This means you will need to amend the code on your website accordingly. Here's a tutorial on how to import and export containers: https://support.google.com/tagmanager/answer/6106997?hl=en
you can actually.. kind of.. Just add the new user as an admin, then remove the old user and voila!