Not an issue
Did this twice, once with paid a fare and once with redemption ex-bkk. If for some reason changi cannot issue your bkk-sin-hnd while u are checking in for your positioning sin-bkk, you can also collect ur boarding pass during at the transit counter at Bkk. Show ur flight details on ur mobile or printout, transit gate staff would let u through and head to the SQ transit desk.