I replaced tens of SCSI tape drives over the years without powering down and have never had a blown card. I would be reluctant to do it on a busy bus (e.g. if your DLT were on the same bus as active disk drives but since these drives are supposed to be on a dedicated bus that shouldn't be a problem). I would power down the external drive and if this is the only device on the bus, I wouldn't bother to terminate it. Opening the bus will immediately trigger a SCSI bus reset but this is considered a normal (if unusual) SCSI event and the underlying protocol is designed to handle it.
The "safe" answer is , of course, power down all components but the "real world" answer is that that would make long uptimes very difficult. Your point about the controller being self-terminating misses the mark. The controller can supply one termination but the bus requires exactly two terminations but if there is nothing else on the bus, it really doesn't matter. As part of the replacement routine, you should do an ioscan anyway to make sure that the new device is "visible".
Operations like this really point out the importance of having a "sandbox" so that you could try out your questionable procedures without risk of clobbering something important.
If it ain't broke, I can fix that.
