Temperature will be fine, definitely don't heat them up.
If you leave them in front of her during the day she'll probably take them anyway (I have yet to have a broody ignore nearby eggs) though do this only if you know which the old ones are to take away at a later date, otherwise indeed swap them in the dark and at room temperature if you'd rather not disturb her.
Having more eggs won't be a problem, she'll be pleased.
So chicks in 3 weeks, hey? Exciting!