As a reminder, ImagePipe never generates a variant in advance: only on the first real request for a precise combination (type, ID, preset or dimensions, format, quality). An old theme and a new theme requesting different sizes never step on each other in the cache: they're two distinct cache keys.
Practical consequence: after a theme change, the old variants (old theme sizes) stay on disk, simply unused from now on. They no longer display anywhere, but in no way prevent the new sizes from generating normally on the next visit. No action is required for the new theme to display correctly.
Clearing the cache stays useful in two specific cases: limited disk space, where you want to reclaim the space taken by old sizes immediately rather than waiting for periodic cleanup; or changing the settings of an existing preset reused under the same name (dimensions, quality, mode). In this second case, the old variants generated with the old settings remain valid in cache until explicitly cleared, which can make an old rendering persist if the preset's name hasn't changed.
To clear the cache: a dedicated page in the plugin's back office lets you clear everything or target specific images, without ever touching the original files in the media library. It's a reversible, risk-free operation, since everything regenerates normally on the next visit.