The Debian PHP 7.0 packages will be co-installable with PHP5, but there's more significant packaging changes than simply the major version bump. While the PHP 5.x packages in Debian have been prefixed with php5 (e.g. php5-cli), for PHP 7 they are also including the minor version number so now packages will be labeled like php7.0-cli or php7.0-fpm rather than just php7-. Similarly, the PHP install directories on Debian will also now include the minor version number.
There are also many other PHP 7 packaging changes for Debian as outlined by this new mailing list post. The first PHP 7.0 packages have just landed in Debian unstable as the developers prepare to debut the support in Debian 9.0 Stretch.