Your game is balanced at the cap. Your new expansion pack raises the cap. How can you let players rise further without completely disrupting things? Nerf everything, then let the players work to get back to where they were. Then launch the next expansion pack.
Do I need a developer quote for something we have seen on so many major expansions? The Lord of the Rings Online™ is the one nearest to my vision. At the end of Shadows of Angmar™, players had quite respectable Block/Parry/Evasion percentages, some avoiding the majority of attacks. Percentages are dangerous things: they automatically scale, and once you’ve reached a certain point, you cannot give more without giving the enemies an ability like “evasion penetration” that feels like a Burglar nerf. When Mines of Moria™ launched, those percentages were all changed to BPE ratings, which were much larger numbers that translated to much smaller BPE percentages. It was an across-the-board nerf to defenses. This, in player relations speak, was done to create the opportunity for future character increases (i.e., work your way back to where you were).
For Siege of Mirkwood™, several other percentages and numbers were changed to ratings. Who would like to guess whether the rating translates to a smaller percentage than it used to be? There is currently a forum debate about whether, because the ratings are level-relative, you actually become weaker when you level up: not just relatively less powerful because you are using your level 60 equipment against level 61 enemies, but weaker against level 60 enemies. I suspect that the math does not work out that way, and it just looks suspicious because the new system is being presented by the same people who made us feel cheated when they gave us free stuff.
That and there were other across-the-board nerfs. The other way of doing it is to leave the players alone but buff all the enemies. Tanks got a boost in the new patch, gaining resistance to critical hits (which is currently bugged to also resist critical heals). Offsetting this (perfectly, if the math works out), all the elite and boss enemies gained buffs to their critical hits. This is one way of discouraging having non-tanks tank: everyone else was nerfed with respect to bosses (and tanks got a boost versus non-group enemies) with what sounded like a buff. This has unfortunate effects on raids with lots of area effect damage, because you can only “tank” a zone-wide attack so much.
There are not many things left to convert to ratings. The next step is to re-norm ratings, and again explain how it raises the ceiling for future increases. Then buff enemies, because players cannot see the numbers behind the DM’s screen. (To a lesser extent, this also applies to changing the rating-to-percentage curve. Players can see that if they look, but it takes active looking to notice that +rating gives an increasingly small +percentage boost.) Then keep making traits increasingly irrelevant by leaving their values capped where they were at level 50, which becomes a rounding error 15+ levels later.