As my webfiltering (Smartfilter) expired and the Mcaffee cost of renewal is
we decided to go with Fortinet appliance that was way cheaper

Can I just use BM as proxy only?
So no authentication (that is to be done on the Fortinet - how I do not yet
know, as I will get it only later this week),
just acting as proxy, so no need to make any changes to the user configs