The system that will be used for challenge modes upon release actually preserves your hit and expertise bonuses, compensating by deducting extra rating from your other secondary stats (crit, haste, mastery, Spirit) in proportion to how much of each you had to begin with.
This sounds complicated, but it should be relatively seamless and handled behind the scenes, with the intent being to minimize the need to reforge/regem for challenge modes.
To use some decidedly fake numbers, let's say I have an average ilvl of 500 and I have the following stats:
Outdoors (Item Level 500)
3400 Hit Rating (+10%)
3400 Expertise (+10%)
6000 Crit (+10%)
0 Mastery (+0%)
8500 Haste (+20%)
When I enter a challenge dungeon, my ilvl scales down to 463, reducing my stats by roughly 30% (rounding for the sake of simplicity). However, the system keeps my Hit and Expertise values unchanged, overcompensating by reducing the other ratings more heavily.
So my challenge mode stats would be:
In Challenge Mode (Item Level 463)
3400 Hit Rating (+10%)
3400 Expertise (+10%)
3356 Crit (+5.6%)
0 Mastery (+0%)
4754 Haste (+11.2%)
(Now yes, if your normal gearset is designed to hit-cap you against raid bosses, you'll effectively be slightly over the hit cap against dungeon targets, so people looking to min/max completely may still have some tinkering to do, but unless you're aiming to beat the realm-best time, the cost of being a couple percent over the hit cap is going to pale in comparison to the benefit of precise and coordinated play.)
Tl;dr version: If you are hit-capped before you enter a challenge mode, you'll still be hit-capped inside. It's magic.