Here is the scenario:

Defendant is arrested for a felony offense. JP arraigns him and sets bond. Defendant's attorney files writ in District Court, seeking to have the bond lowered. (A common practice here.) A hearing is set for that writ. On the same day that writ is filed, same attorney solicits JP in writing, asking him to lower the bond. JP lowers the bond, sua sponte. (Not a common practice here.)

Did the filing of the writ of habeus corpus in the District Court relieve the JP of any jurisdiction over the matter, thereby voiding his decision to lower the Defendant's bond?

An indictment vest jurisdiction in the DC, the writ provides only limited extraordinay jurisdiction. My guess is the JP as the magistrate still has jurisdiction. The JP has probably also been the target of unethical ex parte communication in violation of the Bar rules and Code of Judicial Conduct.

PS Hey Patrick still having nightmares about the video tape collection?

Read and compare these two cases:

Ex parte Chavfull, No. 945 SW2d 183 (Tex. App. -- San Antonio 1997); Ex parte Clear, 573 S.W.2d 224 (Tex. Crim. App. 1978).

From Ex Parte Chavfull: "Reading the three opinions consistently, if another court has set the amount of bail and an application for habeas relief is filed seeking a reduction in the amount, it would appear the district court would have jurisdiction to deny the requested relief or reduce the amount of bail; however, the district court would not have jurisdiction to vacate the bail originally set or increase it. Although there is no evidence in our record of which court set the original bail in the instant case, we hold that the filing of the application for habeas relief invoked the habeas corpus jurisdiction of the 187th, which then had jurisdiction to deny the reduction."

From that I take it that the district court could only lower the bond while the JP court has jurisdiction since the district court has habeas jurisdiction. Also, the JP court could lower the bond should a motion to lower the bond be filed in the JP court (obviously it shouldn't be an ex parte motion!) as well since the JP court has jurisdiction over the case which has not been indicted yet.