Following is available for D7 but not for D8:
Tried following piece of code:
DELETE FROM block WHERE theme = 'NEW-THEME'
INSERT INTO block (module, delta, theme, status, weight, region, custom, visibility, pages, title, cache, i18n_mode) (SELECT module, delta, 'NEW-THEME', status, weight, region, custom, visibility, pages, title, cache, i18n_mode FROM block WHERE theme = 'OLD-THEME')