The main thing you need are support for the Main and Sidebar areas. Depending on how you have used Plain Yogurt, you will probably also want a theme with Header Nav and Header areas.

Personally, I find a theme that looks right for the job, then ask the developer about the areas and page types it supports:
"What page types and what area names are provided (Left/Full/Right and Main/Sidebar etc.)?"

Some theme developers are helpful and already put this information on the theme marketplace page.

Themes may also provide additional areas (Footer/Footer Nav/Second Sidebar) and built ion navigation instead of Header Nav. So you need to know what will happen to these areas if you do not have content for them.

Another thing to look out for is if you have used any C5 customisations of the theme, or for the design of blocks. Different themes have different customisable properties, so they may not map 1:1.

PS: Always make a full backup before installing and enabling a new theme, so you can make sure you can regress to Yoghurt if it gives problems.