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If I have a broody waiting to have hatching eggs I give them other 'sacrificial eggs' to set on till I get the good ones, I call them 'fake eggs' or duds. By this, I mean I just take normal coop eggs and allow the hen to have them to set on while waiting. I switch out the 'sacrificial eggs' every other day or so and just sacrifice them to make scrambled eggs which I feed back to the birds.
If your hen has been setting on eggs for a few days while you were searching for the good hatching eggs you will have to remove them before giving her the new eggs, if you don't you will have problems in a few weeks when her first eggs hatch but the new ones aren't ready yet. Having a staggered hatch over 2 days or so it isn't a big deal, but if there is a 4 or 5 day lag, or more, then the hen may abandon late hatching eggs to take care of the earlier hatched chicks.
So if she is already setting on some eggs you will need to make a decision... sacrifice those eggs and just remove and pitch them.... remove them and place them into an incubator to finish or leave them in place and risk the problems associated with a staggered hatch. The best answer depends on how many days they have already been under her, if you have an incubator available and if you have an area you can brood them yourself.