verb
metamorphose
met-uh-MOR-fohz/ˌmetəˈmɔːrfəʊz/
To change or cause to change into a completely different form.
The insect metamorphoses into a butterfly, emerging from the split chrysalis.

Etymology
“Change” and “form” give the central idea of transformation into a different shape. The same idea extends to a complete alteration of character or nature, not just outward appearance.
In context
when they're both caterpillars and when they grow up and metamorphose into butterflies
More examples
- Over several weeks, the tadpole metamorphoses into a frog.
- The renovation metamorphosed an abandoned factory into a bright community center.
- Under her leadership, the struggling company is metamorphosing into an industry giant.
Forms
metamorphose · metamorphoses · metamorphosed · metamorphosing
metamorphosis noun — A complete change in form or nature.