DISTR BY WX DEVS; A FLC NEGLECTS TO CHANGE FREQ WHEN IT WAS ISSUED RESULTING IN A TFC CONFLICT.

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DURING CLBOUT FROM MSY TO DEN; WE WERE CLRED TO FL350; APCHING AEX; WE RECEIVED CLRNC FROM ATC TO DEVIATE FOR WX. ATC SUGGESTED AND WE ADVISED WE WOULD FLY A 300 DEG HDG. SOON THEREAFTER; WE WERE GIVEN A FREQ CHANGE TO ZHU. I DIALED IN THE FREQ; BUT NEVER SWITCHED IT FROM THE STANDBY DUE TO DISTRS FROM THE APCHING WX. THE CAPT WAS ON THE INTERPHONE TO ADVISE THE FLT ATTENDANTS TO BE SEATED AND I WAS BUSY WORKING THE RADAR. WE ELECTED TO CHANGE OUR COURSE TO THE N TO DEVIATE AROUND WX AND ADVISED ATC. AT THAT POINT WE WERE TOLD WE WERE ON THE WRONG FREQ AND TO STAND BY FOR A NEW FREQ. AS WE TURNED N; I NOTICED A TARGET ON TCASII AT FL350 THAT WAS A POTENTIAL THREAT. THE CAPT AND I OBSERVED THE THREAT TO BE CLOSING AND QUERIED ATC. WE WERE STILL ON THE FREQ THAT TOLD US TO STAND BY FOR A NEW ONE AND I ADVISED HER OF THE TARGET. WE HAD ALREADY ELECTED TO TURN BACK TOWARDS THE W. CTLR TOLD US TO LEVEL AT FL330. WE WERE APCHING FL340 AND SO INITIATED A DSCNT TO FL330. FINALLY; WE WERE GIVEN THE NEW FREQ AND CHKED IN WITH ZHU. ZHU TOLD US TO DISREGARD THE DSCNT AND CONTINUE CLB TO FL350. WE NEVER RECEIVED A TCASII OF ANY KIND; BUT WE SAW THE TARGET VISUALLY AND ESTIMATED CLOSEST POINT OF APCH AT APPROX 8 MI. TCASII SHOWED OUR ALTS AT 1000 FT APART AT CLOSEST POINT OF APCH. ATC MADE NO NOTE OF THE INCIDENT.