Anyone who has ever played this MMO (any MMO really) for more than a year knows this.
Gear is added and/or changed, abilities are added and/or changed, new dungeons are added or changed and mobs are added and/or changed.
It happens near continously. Don't be a one-trick pony and your characters can adapt easily. And even if you were dumb enough to be one-trick pony, DDO at least offers LRs and TRs for you to fix it.