I've got the same issue.
I have 2 T17 42T, one is running with 1 board and the other with 2 boards.
Can i swap one board to the other machine?
If you use the stock firmware you will have to try, there is a combination of voltages stored in the EPROM of each hash board, some will match well some won't, if you use custom firmware, you can simply mix and match as they don't care about the voltage readings.
However, why do you want to do that in the first place? unless you are going to sell the other PSU and Control board, or you are running out of space, it's better to keep 1 working board in 1 miner and 2 in the other, the 3 working boards will run a lot cooler sitting next to dead hash boards.