Usually doctor's won't let you get an IUD if there's a chance your going to have more than one partner (where I live anyway). My doctor prefers only giving them to girls who are married and who have already had children. I don't think it's very wise to have multiple partner because you are putting yourself at risk for severe infections and complications.
It's more about STDs than the IUD. Pills don't protect you against STDs either. That being said taking BC is not carte blanche for you to just go out and nail everything that moves. They're just in their own way telling you to have safe sex. But, in answer to your question, no, you don't need to change your IUD every time you change partners.
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