Migrating from one to another radius server

[Oleg Motienko]

Hello,

We are running billing system with radius auth and acct.
We have to migrate our user database to another billing system but we
can't migrate all users and reconfigure all routers simultaneously.
Presumably we can migrate about 50-100 user of several thousands every
day, and we want to make this process transparent for users.

So, we have 3 radius servers:

oldserver - running  with olddatabase,
newserver - running with newdatabase,
freeserver - runing freeradius for transparent forwarding.

Users enters their logins without @domain and it's unreal to request
all users to change anything in their login configurations.

Is it possible to make such algorithm?
              vvv
Router request auth from freeserver and
1) freeserver forwards request to oldserver and if user is still in
olddatabase, oldserver process request.
2) if user is not found in olddatabase, freeserver forwards request to newserver
3) if user is not found on newserver (so, users is unknow in both
databases) or password incorrect or other error -  freeserver returns
error to router.

If this algorithm is possible, is it possible to forward acct
information same way?

Thanks in advance.

--
Regards,
Oleg