I have RTW and it is E-tix. I am booked NRT-BKK on SQ. Now I want to take TG instead. I called SQ to change to TG flight, but was told (three different agents) that in order to revalidate the E-tix to reflect TG flight, it means the E-tix must be reissued, and thus the fee will be charged.
In the mean time, I also try to deal with TG directly. The same answer was given. Even though RTW is valid for any *A, but in order to switch from SQ to TG, the ticket must be re-issued which involves a fee. (And I had stressed to TG or SQ that no routing change involved, but keeps getting the same answer that the only no fee would be to change the date with same carrier)

Am I missing something? Is this a new rule? I tried to do search here, but could not find similar case.
And yes, one more thing, for E-tix, is it true that no open segment allowed since the E-tix can not be issued with an open segment?