Application migration iRule

I think that it might be because you are using findclass instead of matchclass.

In your example you are saying:

If the [HTTP::uri] is equal to a value that I have stored in my data group, replace that value with the second data group value. If that value is not equal to "" nothing, then direct traffic to pool "old_server_pool-80.

findclass is best used for a Match and Replace.

Searches a data group list for a member whose key matches the specified string, and if a match is found, returns the data-group member string.

http://devcentral.f5.com/wiki/default.aspx/iRules/findclass.html

Try matchclass:

when HTTP_REQUEST {
if { [matchclass [HTTP::uri] equals $::old_server_pool] } {
pool old_server_pool-80
}
elseif { [matchclass [HTTP::uri] equals $::new_server_pool] } {
pool new_server_pool-8001
}
}