Archaic KJV Word
Transfigure
Modern equivalent: transform
What Was Lost
The true nature revealed. The Transfiguration was not Jesus adding glory but Jesus letting His always-present divine glory become visible. The veil of flesh was briefly transparent and the disciples saw who He truly was. Paul used the same word in Romans 12:2 -- 'be ye transformed (metamorphoo)' -- meaning let your true, new nature in Christ become visible in your outer life.
Closest Survivor in Modern English
metamorphosis (scientific term for insect transformation preserving the 'form-change' meaning)
Peak Usage (1611)
KJV Matthew 17:2 -- 'And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light'
Died still used but rare (~1800)
Greek metamorphoo ('to transform in form/to change the outward appearance to match the inner reality/to let the hidden glory become visible') became rare and limited to one Gospel event. The ongoing transformative dimension Paul used in Romans 12:2 was disconnected from the Transfiguration.
What Replaced It
“transform”
Generic change; transfiguration was specifically the hidden glory becoming visible -- the inside showing on the outside
“change appearance”
Surface-level; transfiguration was the true inner reality breaking through the outer form
“glow”
Light effect; transfiguration was the divine nature that was always there becoming momentarily visible