Er... the real question, I think, is what level party are you running through it?
Then tailor to that.
If for instance you want to run an older module for 9th level pcs in 4e, also for 9th level pcs, then you just need to tweak the monster composition of the encounters to make sure they are in the right range for a 9th level party in 4e. Given that a lot of monsters kind of shifted levels, you need to watch this, but it's easy enough to handle with a few minutes prep (VERY few).
Two examples from the sample encounters in Lords of Madness:
Pg. 103, area 2- an EL 8 encounter with a neogi, an umber hulk, 3 orc warrior 1s and 3 goblin warrior 1s.
If I was rewriting this for 8th level pcs, I'd start by setting my xp budget at approx. 1750. I would start with the following:
1 neogi slaver (MM2 166; level 10 controller leader) *worth xp 500
1 umber hulk (MM 256; level 12 elite soldier) *worth 1400 xp
Already over xp budget for a level 8 encounter... but not quite to level 9 (we're at 1900 xp, level 9 would be 2000 xp). So for the orc and goblin minions (aha) we'll add:
2 orc drudges (MM 203; level 4 minions) *worth 88 xp.
Total xp = 1988. Compared to the original encounter we're short four lackeys. If you don't mind changing the EL by 1, to 9, you can add four more drudges (or 1 drudge and build a 4th level goblin minion) and have a level 9, xp 2164 encounter. Perfect.
My other example:
Page 118, area 4- In LoM we have an encounter with a grell necrophilosopher and 3 gnoll skeletons at EL 8. Since grell got a significant level up in 4e, this might be harder. Let's see.
Again, xp budget is ideally 1750 to 1999. We'll use:
1 grell philosopher (MM 144; level 11 elite controller) *worth 1200 xp
3 boneshard skeletons (MM 235; level 5 brutes) *worth 600 xp
Heck, it works pretty much straight across. In both of these encounters we're still in a pretty reasonable level range (level -4 minion to level +3 elite) and I think both would be fun, exciting encounters. And all I did was check the numbers.
So, I'd say it's pretty easy, especially if you're willing to up- and down-level and custom build monsters.