There are tickets that cannot be upgraded or bridged.
Certain, specific, discounted tickets.
Youth Education Series cannot be upgraded or bridged Group tickets cannot be upgraded or bridged.
Travel Agent tickets can no longer be bridged or upgraded. My understanding is/was that the computers did not used to distinguish between types of tickets when people went into guest services. Now, the computer can tell the CM's the ticket type and will simply not allow changes to certain tickets.
Whether or not they have always done it and whether or not they still do it are two different things.
I had clients going down with Youth Education Series and those absolutely could not be changed in any way and when they found out Disney's suggestion was to cancel their Youth Education portion of the trip and repurchase tickets. So, long story short, disney does have some tickets that cannot be upgraded or bridged.
I think people need to read the fine print carefully before buying their tickets.