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ah --- thank you both -- this now makes sense.
The cam  has no bootflag enabled, I assume, because when servicing the cam Canon upgraded the firmware.  The card presumably remains bootable -- because it is the same card that was booting before, when the camera had 2.03.
So, indeed, to reenable ML I have to both "update" to 2.03, and then reinstall ML from a card, so the bootflag will be set on the camera.

I feel much more comfortable doing stuff when I know why I'm doing it.  Thanks.

Walter,
I fear I'm being profoundly dumb, and I don't want to exhaust your patience -- but I remain puzzled. 
I had asked if, with firmware 2.03 on one card, 2.05 on another, and ML on a 3rd, whether it would work as follows:

a. to switch to 2.05 from 2.03  just load the card with 2.05 on it and run firmware update.  after that you could use the ML card, but ML itself wouldn't load.
b. To go back to 2.03 do the same with the card with 2.03, and
c. to run ML after reverting to 2.03 insert the ML card and run firmware update again.

you replied:

a. Nope. Cam won't start.
b. Yes
c. "Firmware update" is not needed.

I think I don't understand why in (a) the "Cam won't start."   If I update from 2.03 to 2.05, I know I can't run ML, but isn't the camera then restored to running 2.05?   It seems to make no difference whether the card I then use (under 2.05) has ML on it or not -- right now the camera has 2.05 and has no problem booting with the card that has my 6 month old copy of ML on it  although of course ML itself doesn't load.

what am I missing?   

Walter --
Thanks again for the response. 
If like that other post I did want to run both 2.03 & 2.05, am I right that you are suggesting that
a. to switch to 2.05 from 2.03  just load the card with 2.05 on it and run firmware update.  after that you could use the ML card, but ML itself wouldn't load.
b. To go back to 2.03 do the same with the card with 2.03, and
c. to run ML after reverting to 2.03 insert the ML card and run firmware update again.

I ask in part because I have my card with ML on it as I had been running it -- if I format that card I lose its configuration -- whereas if I understand, I could revert to 2.03 from another card, then insert the ML card and run firmware update again.   That way I'd be running the same ML as before.   
Do I have it right?
Thanks.

thanks for so quick a response.  I was alarmed by Canon's statement that you can't go back from 2.05.

For the moment I see no reason why I'd want to also run 2.05, as well as 20.3.    although I do have two cards, only one of which has ML on it.   

in post http://www.magiclantern.fm/forum/index.php?topic=9848.msg122566#msg122566 Walter Schulz, responding to a post about ML not running under firmware 2.05, said

I would suggest to use three cards for this. One with Canon's firmware version 2.0.3, one with 2.0.5 and a card running ML (bootable + files + directories).
The cards holding Canon's firmware may be some cheap or small capacity cards.

I don't understand that - could Walter or someone explain?   is the idea that all 3 cards would be independantly bootable by using EOScard.exe?  would the "card running ML" have 2.3 on it?   If the firmware installed on the camera remains 2.05, why the 3rd card with 2.05 on it?   

I sent my trusty 7D to Canon for repair (a new sensor) and it came back with firmware 2.03 replaced by 2.05.  I miss my ML.   Canon says

Firmware Version 2.0.5 is for cameras with firmware version 2.0.3. If the camera's firmware is already Version 2.0.5, it is not necessary to update the firmware. Once the camera is updated to version 2.0.5, it cannot be restored to a previous firmware version.

Discussion on this forum nonetheless implies I can revert to 2.03 by putting that version on the card and using "update firmware."    Can someone please confirm that - thanks.