I am getting ready to start a new campaign that was made for higher level characters. I want to use the campaign but want to lower the monster levels and may want to raise monster levels inthe future. What would be the best way to level/de-level any monster?
4e monsters have a number of critical scaling factors: HP, Defenses, Attacks, and Damage
HP is roughly a function of 8*Level+24. To be exact, compute the distance the monster's current HP is from this (as percentage), convert to the new level, and then apply that percentage. (Beware of early monsters though, they tend to be big boring sacks of HP.)
Defenses are trivial: Level+14 for AC, and Level+12 or NADs. Find the difference from the norms (as number) find the new "Average defenses" for the level, then apply the difference. Beware of fort, entirely too many monsters have a stupid-high fort, which has unfortunate implications for a number of otherwise quite interesting character classes.
Attacks: Level+5 for AC, Level+3 for NADs, with a bit of fiddling on AoE attacks. Again, find the difference, compute new value, apply difference.
Damage should work out to be Level+8. Simply find the average damage for the level, and render it into dice. No need to back-convert.
The big table that this was derived from can be found in the Rules Updates page 45.
It's actually quite trivial to scale that, and skill DCs (make sure you use the ones in the link) to an appropriate level. Remember that grabs now use appropriate moderate skill DCs instead of monster defences.
I'd, personally, just reskin other monsters to add variety and new mechanics. But that doesn't answer the question :)
I do it by the book. It mostly works out. The only thing that doesn't is that there are some powers that get a boost depending on the tier of the character. If you de-level a creature from 11 to 10 and that creature had bonus damage for sneak attack, by the book it would still have paragon level sneak attack despite being heroic. Unless you're trying to be a RAW killer DM, pay attention to these sources of damage and lower them appropriately. Likewise, you should increase them if you level up any monsters.
Scaling monster math is one of the few things the new web-based Monster Builder/Listddi at Wizards of the Coast (with subscription) does.
If the monsters you want to change aren't in the database already, you can either re-skin something there (as suggested by others) or you can download the original Monster Builder (Adventure Tools) to create your own copy of the monster. You can also import it into the web version for scaling and easy access no matter where you are...