An approach for replacement of instructions in an assembly language program includes computers receiving an assembly language program and user selections of one or more classes of instructions. The approach includes computers reading a statement in the program and selecting a class of instructions from the user selections. The approach includes computers selecting a first group of instructions in the selected class and determining that the statement is an instruction in the first group of instructions. The approach includes computers reading a number of statements that match a number of instructions in the first group of instructions including the statement and replacing the first group of instructions with a group of replacement instructions when the read number of statements match the number of instructions in the first group of instructions. Furthermore, the approach includes computers sending the group of replacement instructions to output to update the assembly language program.