Is the salary increase, which occurred by changing the allowance's designation from "engineer" to "technician" without my knowledge, but which my colleague confirmed is my right because I perform the work of both a technician and an engineer, permissible for me to take, or is it unlawful?

There is no impediment to taking the salary increase if certain conditions are met: that the increase is rightfully yours in exchange for work you perform, that the administration's system permits it, and that the employer has the right to change the job title without falsehood or deception. If even one condition is not met, then it is not permissible to take it.