As I understand it the TV is irrelevant to this discussion. You don't tell us completely but I assume your new wiring set up is Cable (Wall) - DTA50 - Splitter - TV and VCR - VCR to Slingbox ?
If so, you don't change the channels using the VCR, that has to be left on Channel 3 (or 4 ?), like the TV. So you move the IR Blaster to the DTA50 and rerun the Video Sources setup to tell it the Composite input now has a new device.