Is it bad to switch crochet hooks when in the middle of a project I lost my crochet hook and don't feel like buying another?

If the replacement crochet hook is the same size as the one you lost, then switching shouldn't be a problem. If it is a lot bigger or smaller, you might see a difference in how tight whatever you are making looks, or it might not work with whatever you are using as well. Totally up to you. Not bad, just can have different effects on what you are doing.