How can I easily port an adventure to another level? Say take Keep on the Shadowfell and instead of it being 1st to 4th level it becames 6th to 9th level.
Restat monsters, check pacing, and reflavour the environment
For all adventures, except for maybe the most recent, the monster design reflects "Pre MM3" design philosophy and therefore... sucks. You'll want to redesign the monsters for the new levels or re-skin modern monsters to match the story. I strongly recommend taking monsters from the MM3 or monster vault in any event. Reskinning can be quite trivial with 4e monster design: simply call a set of powers by different names. If necessary change iconic racial or theme powers to match your desired theme. Even with racial powers, they should be adapted for role as was done for these Duergar.
Especially check the mechanics of traps. They tended to be extremely horrible initially.
The pacing may change, especially from early designs where resources were used at unanticipated rates. You'll want to figure out good stopping points for your party and the consequences of pushing on or holding back.
The environment may or may not change. With Kobold Hall, one of the neater ways to swing is is that the hall is identical, but a new crop of monsters has moved in. It will give your heroes quite a sense of personal power for them to breeze through lower level challenges. If you're taking this route, add in ovbious retrofitting where appropriate, such as thick new locks bolted on doors, flakes of rust around a recently repaired portcullis, and so on. This approach can give an excellent feeling of a "living world" and may encourage your heroes to take the area as their base, if only to prevent yet another group of monsters from taking it over.
If you choose to change the environment, the theme should be roughly: Mundane for Heroic, Expensive and elaborate for paragon and Magical for Epic.
The lower end of each tier should feature run-down and badly out of repair examples of architecture and environment while the higher end should feature good or exemplary design.
Kobold hall at level 1 is an overgrown ruin. At level 5, the internals should be well maintained with a crumbling curtain wall or other outer defence. At 10, it should be a proper fortification, taking cunning and care to infiltrate.
Level 11 should feature well designed architecture enhanced with some magic. Metal walls are now the norm, with hints of the unusual. Magical locks and traps are far more prevalent here, with expected player encounters. As paragon progresses, these semi-fantastic environments should be in better and better repair. A decent level 20 capstone can feature entire structures made out of steel, silver, runes, and gold.
Level 21 is the realm of the fanastic. Adventures in the Astral Sea and other worlds should feature dungeons of force walls, adamantine plates, and other flatly inconceivable items.
In general, the wealth of the party is such that dismantling the dungeon should never be an attractive proposition time-wise. Using these as strong motifs can give the party a clue as to the relative difficulty of an encounter or delve.
DCs, in general, should scale with the party, representing the areas of challenge shifting. While a lower level party could sneak through a cracked wall and have difficulty with a ladder, a higher level party needs to surmount the wall, the wards, and/or the guards. The easy stuff becomes trivial and unmentioned while the environment stops doing players favours.
Converting monsters up or down by 5 or fewer levels is straightforward and listed in the DMG (I think around pg 190, but I'm not positive). You can also swap out monsters pretty easily with premade monsters from the DDI compendium.
Skills are going to be a bit tougher. A ladder has a consistent DC. If there's climbing to be done you can't just say the ladder is suddenly more challenging to climb. You have to change it to something else. A knotted rope is a good substitution if they're climbing a cliff wall. But what if they're climbing up a castle tower. It makes sense that there's a ladder for guards to climb up, but a rope makes no sense.
Furthermore some things that were challenges won't be challenges past a certain level. You can scale the width of a pit as the player's ability to jump increases, but once they can fly that pit is near useless. I don't think this will be much of an issue scaling from 1-4 to 6-9, as flight will probably be a temporary thing granted by a daily power. But you should expect that there are some challenges the PCs will straight up trump and you won't be able to fix them. If they happens just tell the PCs they're awesome and move along to the next challenge.