I think the best way to do it is to benchmark an MM3 monster of the same level and role and do some comparisons. Simply updating damage on the basic attack is a quick solution, but consider other effects. I really do think there's more than adjusting some numbers here.
Here's a Harpy Screecher.
It's from an early period of D&D4 and has those same standards. Presumably, it needs to be updated.
Here's an MM3 level 9 controller.
From what you can see, the stats (hp, defenses, to-hit..) are mostly comparable, except for the damage from that basic attack- which is about 8 (average) from the Harpy and 15 (average) from the Meenlock Stalker. I think the Stalker has a more powerful encounter power (the Horrid Link ability, vice Acerak's Slave), but otherwise the Harpy's screechy pull and slide powers and the Meenlocks dazing and quasi-dominating powers are comparable.
If you really wanted to beef up the Screecher I might add some damage to her basic attack, but it might be even better to give her something like a minor action rechargeable encounter power- perhaps something that chains off of slide or daze, minor action, rechargeable on a 4-6. It doesn't even have to do damage. A fortitude attack that gives a character Vulnerability to Thunder damage or reduced defense against charm attacks.. would also be great.
Causing a vulnerability does the increased damage you want, and creates a more interesting status effect. Another alternate solution- instead of doubling creature damage, consider giving the creature an attack that allows for two basic attacks.