Converting between varchar and float when updating

I have a table with the following schema:
CREATE TABLE foo (bar VARCHAR(32));

Every bar value has a format like a float, e.g. "2.5". Now I want that value
multiplied by two and saved again as varchar. I was hoping to do smth like:

UPDATE foo SET bar = TO_VARCHAR( TO_FLOAT(bar) * 2); -- INCORRECT!!!!

How is that done?

I know that the bar attribute ought to have type FLOAT, but I have to work
with this legacy database. And anyway this table will rarely be updated.

Sincerely, Thomas