I've never seen Fujisaki's other works, but I've heard Hoshin Engi is a great series, but vastly different from Shiki. So I guess whether or not Fujisaki will stay in shounen or move to seinen, would depend on what kind of series he wants to write next or adapt next. I think he'd probably stick with shounen though, since that's what he's done all the way (though I don't think Shiki would have been too out of place in a seinen manga either).
Well, I said Fujisaki possibly can moved to seinen magzine because of Shiki. Compared to his four previous series (Psycho+, Houshin Engi, Sakura Tetsu Taiwahen, and Waq Waq), Shiki is more serious tone and it easily can move to seinen magazine; plus, the computer graphic really suited for seinen manga series.
Furthermore, many authors who started from Shonen Jump moved to seinen magazine as their career gotten longer. Yudetamago, Takahashi Youichi, Houjou Tsukasa, Hara Tetsuo, Katsura Masakazu, Araki Hirohiko, Tokuhiro Masaya, Morita Masanori, Inoue Takehiko, Umezawa Haruto, Kiriyama Kouji, Tsunomaru, and Takei Hiroyuki, all of them started their career from Shonen Jump, but they soon moved to more mature manga as their career reached minimum 10 years. Fujisaki has been mangaka for more than 10 years, so I think it is possible that he moved to seinen magazine.
Although, there is still many mangakas stay in shounen magazine, like Akimoto Osamu and Kurumada Masami.