Recently I've been editing the second book of a series I'm writing called the Warrant Series. The first book (Obligation) was written in past tense. The second book (Cause) I originally wrote in past tense as well. I work in the film industry and much of my time is spent writing screenplays. These are always written in present tense. In fact, Obligation started off as a screenplay that I later adapted into novel form. Therefore, it began its life in present tense. But as the story sat in my mind (for years...) it slowly turned into something of the past. So when I wrote it out in its current form, I moved the story into past tense. This gave Obligation the feel of a chronicle or an origin story; fitting considering the content...
I naturally continued the story in the second book by writing again in past tense. After a year off from the book however, I realized this second story would benefit from the immediacy and intimacy of present tense. I set about rewriting the whole thing into present. It took me about six weeks but I got there and I feel it's better off for it. Hopefully I'll release Cause this summer. I'm interested to see what people think of such a change. Although both books exist in a story continuum they're quite different in content and style. Tense wasn't the only change I made from Obligation to Cause.
Now I'm writing the third book of the series (Discovery). It's different from the first two. This one is mostly present tense AND first person perspective. Generally speaking this third book's narrative structure is significantly more experimental than the first two so I think first person present works in that regard.
Hopefully readers won't be too tripped up by the changes. I know sometimes, when I'm in a groove with a book or series, it can be off-putting for it to vary too much from the formula that established the groove in the first place. It's a risk to make these stylistic changes inside a broader story; but then... I'm not into skydiving or driving fast on mountain roads, so this sort of risk taking is what keeps me alive :-)