As Brendon mentioned, robocopy makes a great tool for this.
Depending if you have extended needs as well, you can look into giving Securecopy a try.
Though for what it is worth, Robocopy will allow you to transfer data (baseline and incrementally) while retaining dates, times, owners, acl's and more.
Depending upon the depth, the amount of data you have, and the size of the window you have to transition this - you can look into breaking down the transfer based upon directory structures (Depending upon how you have your data laid out, multiple trees, etc)
If you also happen to be doing any host-data in addition to the Celerra, you should make sure that the ACL's assigned do not consist of any 'local group' type of security as this can be a challenge as part of your migration process. If it is all domain global/universal group membership as far as security goes, this likely won't pose a problem.
Good luck Ed!
Christopher