Do the sizes just get adjusted with the new layouts? Or do things get shifted around?
If your existing ads are hard coded into your content (this just means that the html tags for the ads are part of the content html and not the template layout html), then your existing ads will be plugged into the new Ezoic template, in a similar position to where they were before.
You then simply decide whether or not to include any extra ads, that can come with the new Ezoic layouts. If not, you just turn off ads in the Ezoic dashboard.
How do I control how many extra ads are shown in addition to my existing ads?
Click 'Settings' at the top of your Ezoic dashboard and then 'Max. Ads Per Page' in the Basic Site Options.