Well, the pros would be that the ethnic groups would learn other cultures, like how we established Africans into America ages ago, and now they are a part of our culture in whole! (This is from an American's PoV.) Another pro would they would reproduce other ethnicities with the other ethnic group, therefore, expanding the ethnic group tenfold! A con would be that there is the chance the ethnic group could be diminished in whole, and only the reproducings of the changed group would be left, like how you don't see as many dark African Americans now, more lightskin ones. Another con is that the religious practices of that group could be eliminated, where only a select few now only practice it, instead of the whole ethnic group.
I hope this helps!