I already owned a property when I purchased my buy-to-let property last month. If I sell my current main residence and move to the buy-to-let property, can I request for SDLT higher rate refund?

You may not be eligible for the SDLT higher rate refund. To eligible for the refund, the new property purchased has to be your new main residence. The intention at the time of purchase of the new property is important. You have purchased your second property for investment purposes hence you are not replacing the old main residence with the new main residence at the point of completion.