According to the official blog, Kamiya Akira was dismissed from the voice actor of Mouri Kogoro in Detective Conan. He didn't reveal the detail of the reason for the dismissal but he said he had only himself to blame. He explained that "There was an issue in the contract. It's a matter of faith, justice and sincerity (真・義・仁)." He apologized to the fans, other seiyuu and the staff involved.

Kamiya Akira has been playing the role since 1996. His last Kogoro will be aired on October 10th.

Sources:
Kamiya Akira's official blog
Asahi

Updated on Sep 19th
According to Mainichi News, the reason for the dismissal was a discord between the anime producer and Kamiya Akira regarding the guarantee fee. Kamiya has been negotiating with TV companies and the anime producers to raise the guarantee fees of seiyuu, whose salaries are significantly lower than those of the normal actors. Yomiuri TV, the producer of Detective Conan, was perplexed with Kamiya's premature disclosure because they were planning to announce the change of the voice actor in Weekly Shounen Sunday to be released on October 28th.

Source: Mainichi

Updated on Sep 20th
Kamiya Akira explained the reason in his blog. One of the staff of the production company leaked confidential information of Kamiya's contract with the producer in a blog (probably mixi) and defamed him. Kamiya and the producer were about to reach an agreement but the negotiation broke off due to the leak, which was distorted with ill intention. Kamiya asked a man to intervene but the producers fired him in the end.

Kamiya begged fans to keep supporting Detective Conan.

Source: Kamiya's new post in his blog

Updated on Oct 28th
According to the official website, the new seiyu of Kogoro is Koyama Rikiya.

Source: Web Sunday