The AF/KL 'merger' was a 'merger' with AF effectively buying KL which was on the verge of going to the wall. The forums were initially merged, but owing to problems between members, and the fact the brands were persisting, the forum were moved into the current set up.
Additionally LH bought Swiss, and the Swiss forum was merged into the M&M forum, but maintained on a sub-forum basis because Swiss was operating as a different brand. So there is absolutely precedence for acquisitions to be treated like that.
I don't think mergers and acquisitions have been treated differently. I think before the KLM/AF merger, just shuttering forums and pushing groups of airline users together was seen as not a problem. However the open warfare that resulted from that between the two user groups made people realise that sometimes it wasn't as simple as that, so we have moved away from the model which was used for Canadian and TWA to deciding more on a case by case basis what works best for the user groups. Both of those were some of the first programme joinings we encountered on FT.
I think it does need to be considered on a case by case basis, and there hasn't been much discussion about what happens. Or at least not openly - and the users need to be kept informed and consulted.