Another solution which, in my opinion, is easier to read would be:
UPDATE test
SET something = 1, field = IF(condition is true, 1, field)
WHERE id = 123
What this does is set 'field' to 1 (like OP used as example) if the condition is met and use the current value of 'field' if not met. Using the previous value is the same as not changing, so there you go.