Once you have upgraded to version 8 or 9, you can have existing clients that will work with an 8 or 9 media agent. However, any new installations would need to be done using the same version your CommServe has been upgraded to. These new clients would need to be backed up using a media agent that is on that upgraded version, which is why media agents would be the suggested second part of the upgrade after the CommServe. A mixed environment is supported provided these considerations are taken into account. In version 8 and version 9 you do have the ability to push upgrades out in the same way as updates (with the exception of clusters and sharepoint agents), as described below.
In 8.0 the Update Cache has also been incorporated to hold the CV software allowing for upgrades to be pushed from the CS GUI, with some exceptions, such as clusters and SharePoint. The directions from the link below starting at step 3 will instruct you on 2 ways to populate the update cache with the CV software. I would suggest using the manual method of creating the directories and copying the software. Please note that the DVD(s) are pretty big in size so make sure you have enough room or relocate the Updates Cache in the Automatic Updates settings. Once the software has been copied over, you can then follow the instructions to push out the upgrades.
***Make sure you are using the latest installer package (Jan 2010) which can be downloaded from Maintenance Advantage.***