Uni Directional & Bidirectional on Load Tapchanger

This question depends on how it is possible to move the tap changer through the possible choices. A unidirectional changer would go up-up-up (or down-down-down) until jumping back to the bottom (or top). A bidirectional changer would allow you to go up/down from any position to the next position in either direction. (Probably the better overall choice.)

Think (tho' it's pretty unclear ) that OP's talking about using a step-down transformer (with auto tap changer) as a step-up transformer.

None of the above. OP is referring to the type of automatic system that controls the reaction of the tap changer to current flow in addition to voltage changes. Traditionally tap changing transformers were at the end of radial transmission line with no local generation so it was just necessary for the control to respond to the diurnal voltage changes that took place as the rest of the grid voltage fell during the day as the load increased, and rose at night as industry and consumers reduced their demand. All current always flowed from the high side to the low side.

Fast forward to modern systems where there may be local generation and multiple connections for a load center such as a town and/or industrial facility. Now it's possible for power to flow in multiple directions through the transformer(s) and simply controlling the tap position based upon voltage isn't good enough, the controller must be capable of distinguishing between voltage change from the grid and/or the effects of local generation, hence the term bidirectional.

The answer to OP's question is it depends upon whether the controller is designed and set up for the conditions where it is installed, the OLTC itself doesn't know or care which way power flows through it, except for the additional number of tap changes it goes through and their effect on the maintenance and the useful lifetime of the mechanism.