An update conflict occurred due to another user process deleting the record or changing one or more fields in the record

Thanks Kranthi.

I had tried the tablebuffer.reread() but it is making the code to rerun again and throwing an error/message that the record already exists in the ProjCostTrans table. Calling a reread() method on the table definite may be a solution and I might be doing something wrong at the same time. However, to make this simple and avoid the confusion I just created a local buffer for the LedgerJournalTrans and selected the record based on the current LedgerJournalTrans.Recid and then updated the local buffer has solved the issue. Which looks something like this

select firstOnly locLedgerJournalTrans where locLedgerJournalTrans.RecId == _ledgerJournalTrans.RecId;
locLedgerJournalTrans.selectForUpdate(true);
//_ledgerJournalTrans.reread();
locLedgerJournalTrans.DefaultDimension = projCostTrans.DefaultDimension;
locLedgerJournalTrans.doUpdate();

Why would calling reread() on ledgerJournalTrans will insert a record in ProjCostTrans? That is something you have to investigate. Try by adding a break point to ProjCostTrans.insert() method.

Your solution is OK. I would rather suggest you to use \Data Dictionary\Tables\LedgerJournalTrans\Methods\findRecId instead of new select.