The current 73 appointed senators will probably leave office earlier than expected after a joint parliamentary meeting voted to cut short their term.
Under the current charter, their six-year term will expire in May 2017.
On Wednesday night, the joint session debated Section 12 of the charter amendment bill seeking to change the composition of the Senate.
Section 12 allows the current appointed senators to remain in office until their term expires in 2017.
However, after a four-hour debate, the meeting voted 380 to 8 against Section 12, which has been proposed by the charter amendment scrutiny committee. The meeting then voted 333 to 1 in favour of a proposed change to the section.
The change was proposed by Pheu Thai MP for Roi Et, Niramit Sujaree, who proposed that the term of the current appointed senators ends as soon as the new batch of elected senators assume office. The term of the current elected senators will expire on March 2 next year.
Appointed Senator Somchai Sawaengkarn told the Bangkok Post Thursday that the six-year term of the current appointed senators expires in May 2017.
But if and when the bill passes parliament, their term will end straight away after the Election Commission (EC) calls a Senate election next year, Mr Somchai said.
If the bill passes, the EC is expected to call the Senate election within 45 days after the term of the current elected senators ends on March 2.
The charter amendment bill requires all 200 senators be elected.
Pheu Thai MP Samart Kaewmeechai, who chairs the charter amendment scrutiny committee, said if and when all the senators are elected and assume office, the appointed senators will no longer be necessary.
Pheu Thai MP for Samut Prakan Worachai Hema, who sits on the scrutiny committee, said that in the event the charter amendment bill passes parliament, retaining the appointed senators alongside the new 200 elected senators would violate the constitution.
The joint session on Wednesday night also voted 337 to 35 to delete Section 13, the final section of the charter amendment bill, which is related to Section 12.
Senate Speaker Nikom Wairatpanich, who chaired the joint meeting, said the meeting will resume on Sept 27 to vote on the bill in its third and final reading.
Pheu Thai MP for Nonthaburi Udomdej Rattanapian has admitted that the fate of the charter amendment bill will hang in the balance if the Constitution Court accepts a petition against it.
The Democrat Party plans to petition the charter court to rule if the charter amendment bill, which passed its second reading in parliament on Wednesday night, violates the constitution.
"If the bill is taken to court, the chances are 50/50 and anything could happen," said Mr Udomdej, who is adviser to the government whips.
He said the petition by the Democrats was not beyond expectation as it was their last hope to stall the bill through the charter court.
But Mr Udomdej said he hoped the court would not deliver rulings which run counter to public sentiment.
Opposition leader Abhisit Vejjajiva Thursday reiterated that the second reading of the bill has violated the charter, with the opposition having been denied the right to debate.