2003/2010 co-existence - is it safe to create recipient policies under 2010?

Question

I'm pretty sure this won't affect anything, but just want to be 100% sure.
Exchange 2003/2010 are in co-existence happily working away
The existing Exchange 2003 recipient policies are a real mess
To save having to fix them all up, the plan is to just create new ones under Exchange 2010 and scope them to apply for users on 2010
When all users are moved to 2010, the old ones can be removed which saves having to upgrade them to all to 2010 format which is another bonus
Will creating new recipient policies under 2010 apply to recipients on 2003 or force the 2003 policies to run again somehow against existing users, or can recipient policues under 2010 be set to only apply to users in 2010 using a custom attribute for example
and when users are moved to 2010 with the custom attribute the policy can be enforced at that time?  All the current polices are set to applied = false under 2003.

Answer 1

I have experience with messing up in this scenario :) in a 20,000 user environment.
E-Mail address policies are shared across the Exchange Organization so you will see ones created in 2010 in 2003, although you won't be able to edit those.  Having the 2003 policies set to false means that they haven't been applied in some time and
its best not to touch during the migration.  My first step here (after making a mistake) is to Disabled the Automatic Email Update for all mailboxes.  I can look up how I did this on a mass scale a little later.
I then created the policies in 2010 to use a custom attribute, as you had suggested, and when mailboxes were migrated over, I made sure those accounts had the Attribute and then checked the box to auto apply the address policy.
Someone else may have a better answer for you, but this is how I saved my butt in a larger organization.
Jason Apt, Microsoft Certified Master | Exchange 2010
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