For macOS 10.10 and later only. This script will simply remove every track in a single Smart Playlist selected in iTunes (if possible*) which will oblige it to repopulate, ostensibly with different tracks. Works effortlessly when assigned a keyboard shortcut.
*Note that tracks cannot be removed from a Smart Playlist which already contains every track of the library meeting its criteria--it would just repopulate with the same tracks. Smart Playlists with the "Limit to" and "Live updating" criteria in use are more likely to be "refreshable".
Also see Refresh Smart Playlists, which works with a batch of selected Smart Playlists.
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Remove iTunes tracks disassociated from files
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Resample selected tracks' artwork to user-set size limit