There is no physical video format conversion on the Bolt transmitter's SDI loop out.
The same video format that comes in to the TX over HDMI or SDI will be sent on the looped SDI output on the transmitter.
However, there is video format conversion possible between the TX and RX by using the video format scaler on the Bolt receiver's output options.
This is possible on 2nd and 3rd generation Bolts, as well as the Bolt 4K series*.
*The Bolt 4K LT RX does not have format output conversion capabilities