Why would you think the initrd image had been changed?
No upgrade touches the kernel or the initrd image. You must manually install new kernels through Synaptic, and then the initrd image for each new kernel will automatically be created specifically for your installation. If you change hardware or alter the partitions, causing boot problems, you can manually run the mkinitrd application to then create a new initrd image that takes into account the changes. It won't be done automatically except for the original kernel installation.