Disney has created a program similar to Warner Brothers’ DVD to Blu program. Instead of doing an exchange with the company of a DVD for a Blu-ray for a cost, you get a voucher worth $8 off of the cost of a Blu-ray edition of the film. Warner Brothers, last time I looked through the list, had you send your physical DVD to them and you paid a small amount and they’d send the Blu-ray to you.
If you would like to upgrade either, or both, of your Disney produced Chronicles of Narnia DVDs to the Blu-ray release, you can do it on their new website: Disney DVD to Blu-ray Upgrade Program.