Here is something that you can do:
Start with a SWiM output fully loaded.
Change all receivers/tuners to the DSWMLNB setting.
If the receivers have actually changed, you will lose one or two tuners.
Channel 1 is the same for "all" @ 974 MHz.
Channel 9 is 1790 MHz, while channel 14 [DSWM] is 1637.39 MHz
An interesting experiment, and one I'll try when I have the chance (though I have only 7 H24s) but I don't think there's a possible scenario where you would lose tuners. Here's why: we don't know how SWM assigns channels to receivers.
There are two possibilities I can think of. It assigns the receiver a SWM channel number (2-9 for ASWM, 2-14 for DSWM13) or it assigns the receiver a specific frequency. Let's assume for the purpose of illustration that setting the receiver for a DSWM LNB makes it really "believe" (act as if) it is connected to one.
If it assigns by channel number, and the receiver was for example assigned channel 3 by the ASWM, since it thinks it is connected to a DSWM it tunes 1076.06 MHz where both DSWM channel 3 and ASWM channel 2 are located. Obviously, it would be unable to change channels since it would be tuning a frequency assigned to another receiver. Since I was able to change channels when I tried this, it is impossible for the receiver to both be assigned by channel number and "believe" the DSWM LNB setting. Thus in this case your test would not lose tuners, because the receiver doesn't the setting.
If it assigns by frequency, then the receiver would work perfectly whether or not it "believes" it is connected to a DSWM. It wouldn't know/care that the channel spacing is different, it would just tune to the frequency provided to it. In this case, your test also would not lose tuners, but unfortunately that would tell us nothing.
So can we tell whether SWM assigns frequencies or channel numbers? I can think of a way, but can't test it myself since it requires access to a DSWM. You'd connect a receiver that has nothing in the firmware for the DSWM and see if it works (either one with old firmware, or perhaps a H20 which "probably" doesn't know about DSWM since it has no setting for Slimline-3DS in current firmware) If it works, and can change channels, SWM must be assigning by frequency. If it fails, it is either assigning by channel number or the firmware refuses to work with a SWM that reports a rev it doesn't recognize.