This card can be searched by A Cat of Ill Omen from your deck.
It manipulates the Monster Type on the field which certain themed decks may benefit. E.g. Declare Thunder-Type then use Makiu, the Magical Mist and Declare Dragon-Type to increase Buster Blader's attack.
Your opponent might be running a deck that benefits with the Monster Type that you declared.
Tips, related cards, skills
Destroy them all!
Declaring all monsters into a certain type, will allow you to use cards that destroys all monster of that type. Cards like Eradicating Aerosol and Breath of Light are examples of such cards. It's important to remember that using these cards will also destroy your own monsters.
There are a lot of potential combos by using DNA Surgery, but its main problem is that you can only obtain 1, and getting it early in a duel can be a problem. You can get 2 copies from leveling Ishizu, which can be used to get DNA Surgery from your deck easier.
Similarly, "Tuner", "Synchro", and "Fusion" are not a Type either.
Zachattack 21days ago
I think this card could be a key player in defeating plant decks. Because any effects of plant monsters traps and spells relies on plant type monsters, so when you change the type of all monsters on the field it could be a quick way to cut those plants down to size!
No, the newest one played would take precedence so if DNA was on the field as Insect and the other player played DNA surgery and called out Dragon then Dragon would be the type canceling out the first DNA