will stepdown transformer can be used as stepup transformer in three phase supply?

Answer 1

A step down TXFMR can certainly be used as a step up. However, generally step up transformers are Dy and step down are Yd. By using a Yd transformer as a step up there will be no ground fault protection on the line.... so if there is a ground fault on any phase only, circuit interruption will not occur. If the circuit is a cable, a ground fault rarely occurs without it affecting at least two of the phases, thereby becoming an overcurrent fault and either will blow the fuses or trip the CB at the transmission end feeding the fault.
I would recommend that you install a grounding transformer on the step up end to ensure that the line has ground fault protection.

Source: Electrical Engineer (ret.) with 45yrs+ experience

Answer 2

If you can reverse the direction of the overall flow, a step down and step up transformer can be switched.