In our previous work we have introduced an analogue of Robinson-Schensted-Knuth correspondence for Schubert calculus of the complete flag varieties. The objects inserted are certain biwords, the outcomes of insertion are bumpless pipe dreams, and the recording objects are decorated chains in Bruhat order. In this paper we study a class of biwords that have a certain associativity property; we call them plactic biwords. We introduce analogues of Knuth moves on plactic biwords, and prove that any two plactic biwords with the same insertion bumpless pipe dream are connected by those moves.