Migration features for painless upgrade from older Centos to Centos 6.

Please provide account migration features which can be applicable for upgrading server from old centos to centos 6. This request is based on forum thread http://forums.cpanel.net/f49/how-upgrade-server-centos-5-6-w-o-much-downtime-pain-377781.html

Possible features that maybe useful in that (these maybe not optimal as I am not expert in all details of inner cpanel workings):

1. Top level IP migration from one server to another. Which could migrate all related accounts automatically. (Preferably with smallest downtime as possible. And because IP will be same DNS does not need to be changed.)
2. And/Or documentation for performing upgrade like that with existing methods.

Low level:

1. restorepkg without changing DNS records for client domains (that is source of many errors which is painful on mass migration).
2. restorepkg force IP to really force it (now it can deny force ip by some reasons). This is need when new server have one shared IP but migrated users have another shared IP.
3. restorepkg to not clean subdomains if user directory is already filled with content and cpmove is packaged with --skiphomedir. Btw, 'Account copy' have some options to optimize transfer but they are almost not explained and very puzzling how to actually do them properly in real situation of mass migration. I want to pre-copy /home with rsync first (before shutting down client sites) and then faster rsync just before restorepkg. But now restorepkg will clean subdomain directories. This forces me to do rsync again after restorepkg and I'm not sure that user's homedir on new system could be rsynced like this from old system without bad side effects.
4. and detailed documentation how to speed-up pkgacct/restorepkg cycle as much as possible to minimize clients sites downtime on mass migration. And how to keep IP and DNS untouched.

Overall I conclude that cpanel is not designed for mass migration except for simplest cases of account copy and multiple account copy to new server with obligatory IP and DNS changes. (Or it's not documented). What I need to do is migration of users without IP and DNS changes, because changing DNS is very problematic. (There is many custom DNS configurations and cpanel is crude in changing it to what it 'thinks' right, plus changing DNS is downtime simply because of ttl issues, and cpanel have default ttl of many hours. Also not always is possible to change DNS at all, there is always several clients like this on any server).

If scripted mass migration (w/o IP & DNS change) will be possible, then it will be possible to just migrate all users to other server in one night, then upgrade first server to centos6 by reinstall (which is only supported method of upgrading to centos 6) next day, and migrate them back other night. Without this, now, upgrading older centos (4,5) systems to centos 6 is almost impossible or very long, error prone, and tedious process.