Uniforms were issued with new buttons. They would have appeared new, so new buttons on a repro uniform are perfectly fine. This topic comes up on occasion and turns into a whirlwind of time wasting back and forth about what chemicals, human waste, and other substances can artificially age a button. Do yourself (and the moderators here) a favor and age them the way they did - wear them in the field or polish them in garrison.
Moderator and Sewer of Historical Clothing and Tall Tales
"But our opportunity to learn and grow, to communicate the richness of the lives that have gone before us, that does not change. We do not outgrow it. It does not tatter and fall apart in our hands..." -Mrs. Terre Lawson, 2010