Well, it has been a few years (or more) since I made any changes to my Tinkerer's Daughter covers. I've been pretty happy with them for the most part, and I think I can credit the ongoing success of this series in part to the covers' abilities to draw readers in. The first step to selling a book is, after all, enticing someone to pick it up.
I think it helps to revisit old covers, descriptions, and maybe even the stories themselves from time to time just to make sure improvements aren't warranted. There's always room for improvement and depending on the situation, it may or may not be worth the time and trouble. In this case, the cover to T1 was decent. At the time I published it, I think the art was comparable in quality and appeal to most of the market. The market has grown since then -it has become more sophisticated, more mature, and the art has as well. I've been thinking for a while now that this cover needed a little sprucing-up, and it just so happened that I had some time on my hands this week.
Here's the original (or at least, most recent) cover: