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Hi rangechicks! Great advice so far.
I wanted to elaborate a bit on what bamadude and jerryse said.
When the roosters breed a hen the sperm will travel up her reproductive tract and will be stored in sperm storage tubules (SST) in the oviduct. They act as tiny vessels that will store sperm for the hen so that a single mating will allow her to fertilize successive eggs she produces for a clutch. It is not know how long the sperm will stay there, but generally the recommendation is to wait four weeks. Many variables may contribute to length--age of rooster, breeding frequency, number of other hens he is breeding, type of comb (rose combs are less fertile) etc. Remember that it takes about 24 hours to produce and egg so that first egg produced after the rooster switch for sure would be from the old rooster.
It is well known that there is a phenomenon called last-male precedence; the last male that mates with a female tends to have his sperm used and is more successful. It has been postulated that the sperm packs into the SSTs and so the first rooster the hen mated with will have his sperm at the bottom of the vessel where the most recent one will have his sperm packed in on top and thus it will be used first. So the hypothesis is first in last out on the sperm. There is one study
http://www.reproduction-online.org/content/123/1/79.full.pdf that looked at this segregation as an explanation for the last-male precedence by inseminating hens with stained then unstained sperm and then looking at the sperm in the SSTs and expecting to find the stained ones at the bottom and the unstained ones at the top. This was not the case--various SSTs tended to have only one or the other type of sperm with 4% having a mixture. So this did not support the layering hypothesis. There were some problems with the study but I thought it was interesting none the less.
There is also anecdotal reports that a chick was produced from a pairing that took place well before the 4 week window, so that 4 weeks is not a hard and fast rule.
I think that what happens is that (in spite of the study), the sperm from the old rooster is stored at the bottom of the SST and kept safe there. If the new rooster fails to breed the female for a period of time or becomes infertile, that old sperm may make its way to the top as the newer sperm is used up, and fertilize the egg unexpectedly.
My advice mirrors jerryse: use up the old rooster's sperm prior to introducing the new rooster to make sure that there are no stray bullets to hit the bulls eye, so to speak. If you fail to take this step you may be surprised with a chick that pops up from the first rooster.
This was 2 weeks ago, we have gotten more over the past 2 weeks
