UNAUTH RWY ENTRY OP AFTER FREQ CHANGE PREMATURE.

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I APCHED COLUMBUS; IN (BAK) AND WAS GIVEN INSTRUCTIONS TO ENTER A R BASE FOR RWY 32; COMMUNICATING ON TWR FREQ; 118.6. AFTER BEING CLRED TO LAND ON RWY 32; I APCHED AND LANDED. COMMON PRACTICE IS TO TURN L ONTO RWY 23 TO TAXI TO TXWY E; TO THE RAMP. AS I APCHED THE INTXN TO RWY 5/23; I HAD NOT YET RECEIVED TAXI INSTRUCTIONS FROM THE TWR; AS EXPECTED. AT THIS POINT I SWITCHED FROM TWR FREQ TO GND ON 121.6 AS I TURNED ONTO RWY 23; AND ASKED IF IT WAS OK TO BE ON THIS RWY. CTLR'S RESPONSE WAS HE HAD AN ACFT ON APCH TO RWY 5; AND EXPEDITE MY TAXI TO THE RAMP. HE THEN ASKED IF I HAD COPIED HIS XMISSION TO OTHER TFC? I REPLIED 'NEGATIVE.' AFTER TAXIING TO THE RAMP; I ATTEMPTED TO CONTACT GND TO GET A CLARIFICATION AS TO WHAT HE HAD BEEN REFERRING. SINCE THE TIME WAS NOW ABOUT PM05L; AND THE TWR CLOSES AT PM00L; I WAS UNABLE TO RAISE HIM ON THE RADIO. ABOUT THAT TIME I SAW AN ACFT LAND ON RWY 5. I AM ASSUMING THAT THE TWR HAD COMMUNICATED WITH THE LNDG ACFT JUST AFTER I CHANGED FREQS; AND JUST PRIOR TO HIM REPLYING TO ME ON GND FREQ. I BELIEVE I MADE A COMPOUND ERROR: I SHOULD NOT HAVE CHANGED FROM TWR FREQ UNTIL INSTRUCTED TO DO SO BY THE TWR; AND I BELIEVE I SHOULD NOT HAVE PROCEEDED ONTO THE OTHER RWY WITHOUT HAVING BEEN CLRED TO. I HAVE SINCE CLARIFIED THAT I SHOULD REMAIN ON THE TWR FREQ UNTIL INSTRUCTED TO CHANGE. I HAVE NOT BEEN ABLE TO CONFIRM THE NEED FOR CLRNC TO TAXI ON THE OTHER RWY WHEN I WAS NOT GIVEN TAXI INSTRUCTIONS UPON LNDG. WHEN TO CHANGE FROM TWR FREQ TO GND UPON LNDG; AND WHEN TO CHANGE FROM TWR FREQ TO DEP ON TKOF HAVE ALWAYS BEEN VAGUE TO ME. IF I ASSUME I AM SUPPOSED TO BE INSTRUCTED TO CHANGE FREQS; THEN IT IS NOT PROMOTED BY ATC; AS I HAVE FREQUENTLY ENCOUNTERED; THUS CONTRIBUTING TO MY CONFUSION. IN THE FUTURE; I WILL REMAIN ON TWR FREQ; AND WILL NOT CROSS A RWY WITHOUT TAXI INSTRUCTIONS; WHICH I WILL ASK FOR IF I HAVEN'T RECEIVED THEM PRIOR TO XING A RWY.