Well,actually here the seme is the replacement,but this manga is really nice!
Empty heart
http://www.mangago.zone/read-manga/empty_heart/
i think this one ... it has the same theme as replacement/substitute seme ...
http://www.mangago.zone/read-manga/tokidoki_sekai_wa_utsukushii/
Sasayaku no wa Sono Yubi
Ashita Kimi ni Ai wo Sasayaku
Kyou Anata ni Wakare wo Tsugemasu
two one shots prequel and sequel
a yaoi manga where the uke becomes a replacement at first then the seme eventually realize his true feelings, somethin like tht ??? Plsssss heeelllpppp :))