Hi,
Thank you for posting in Symantec community.
I would be glad to answer your query.
Asked questions answer depends upon the network structure.
Total number of clients in the network? Total number of sites including WAN links?
I would like to explain in simple term. You will have to use management console (SEPM) and deploy SEP anti virus package on all the workstation where Macfee is installed. SEP anti virus package can uninstall Macfee and install SEP on that in one go.
Check this article to know more about supported products.
About the third-party security software removal feature in Symantec Endpoint Protection 12.1 RU1 MP1 and later
http://www.symantec.com/docs/TECH178757
Security products that are not in the supported products list can be removed using the SEPprep tool.
Personally, I don't see any risk with the migration process however precautions should be taken.
Prior to implement in the network test the SEP client package with all the VM images if there are multiple. Check working of all installed applications and performance etc...
I hope Symantec Endpoint Protection (SEP) will make life more easy .