Hi,
If notams are not available for some reason for alternate A (the declared ATC flight plan alternate), then is there an issue with nominating alternate B, (WITH Notams , met and sufficient flight plan fuel) before departure; without having to re-file a ATC flight plan.
There is no problem with your procedure. It saves all the trouble of refiling the same flight plan details again for the sake of one change. If you did have to divert, then you'd tell ATC on the radio of your intention.
If you chose to divert & subsequently had a radio failure before telling ATC of your revised alternate, then it would cause them some confusion. So it might be prudent to tell ATC of your preferred Alternate, before your radio failure, if you think you may have to divert. It's like "Take these tablets 5 mins before your head ache comes on."