At DL there are very few resellers, and those few are travel agencies. But there have been many resellers at WDW for years, and it was long used as a way to get a discount on tickets. Like you, I suspect that they are finding all the ways to prevent discounts and to encourage direct sales.

Another rumour is that bridging will no longer occur, except for tickets bought from AAA. In late 2013 the Disney/AAA relationship was in jeopardy, and I wonder if the contract was renegotiated to give AAA priority above ticket resellers.

I'm not sure what you are trying to say.

It IS a big deal for people. To keep it simple, imagine if someone bought a 5 day ticket for $400 which Disney was selling for $450, and wanted to upgrade to a 6 day ticket. If (like at DL) Disney now only gives credit for the actual price paid, that customer will end up paying $50 more than they expected to pay, after making a legitimate purchase.

If you are are saying that Disney should charge them the extra days only, and not consider that they only paid $400 for a ticket which Disney sells for $450, then there is no change from the current policy.

I can see this happening for purchases made after a certain date, but there are several sources reporting differing experiences, including people not being able to add a day at all, and being told that they have to buy a one day ticket for $99 instead of upgrading their ticket.

There has been no official announcement on this, and I frankly don't expect that we will see much if any official information if this does become some new policy.