One of my favorite posts dealt with updating pre-MM3/Dark Sun monsters. Its like nails on chalkboard when I hear someone saying that the old monster books are worthless. Sure, the Monster Vault is the peak of 4e monster design, but part of that is that you get tokens and an adventure. Its not very helpful to just tell someone to go and buy another book or box set. Updating the old monsters does not need to be akin to filing your taxes. You can add adjustments completely on the fly, without ever putting pen to paper, and revive all of your old books.
The main problems with old monsters come down to low damage, low accuracy, and (for Elites and Solos) susceptibility to debilitating conditions. Part of this can be handled by limiting the power creep at your table. I advise using some form of inherent bonuses a la DMG 2 or Dark Sun as a start. Even then, the monsters will still need help. If you do nothing else, follow my instructions on increasing damage.
Lets update and edit the old post's tips with some new notes added. Keep in mind that these tips do not apply to Minions; post-errata Minions should do 4+1/2 their level damage (minimum 1) and you should adjust them separately. Anyhoo:
2. ATTACK BONUSES: Adjust monster attack bonuses up to at least Level +5 vs AC and Level +3 vs NADs. If any attack bonuses are already higher than this, such as an Artillery's RBA, leave them be. Brutes will be the main ones you will need to look out for.
3. MULTIPLE ATTACKS: If they do not have one already, give the monsters a "double attack", a single Standard Action that allows them to make two RBAs and/or MBAs.
4. RECHARGING: Lower any recharge numbers by 1. Encounter powers become "recharge 6".
5. CONDITIONS: Solos and Elites are always immune to the stunned and dominated conditions. To be honest, I would just remove stuns from your game altogether.
6. LAST DITCH EFFORTS: If they still look too weak or don't feel tough enough, let them Free Action attack when they are bloodied and/or when they die.
If you commit these simple tips to memory, you can pick up any old copy of Dungeon, any dated adventure, any old Game Day relic, or any early splat book and instantly breathe life into it. Its actually fun to pick one up, dust it off, and practice in your mind. Its very empowering because you can do it in-game, just eyeballing it. It becomes very natural and automatic if you try it a few times, to where you can look at any stat block and immediately adjust it without much thought.
Anyways, as I said above, I like advice that is practical, easy to use, and very specific. Try this in your 4e games and I think you will find that your old books are still awesome, and you didn't even have to write in the margins.