Transparent Proxy Feature

Hello,

I am new with Fortinet family and currently evaluating Fortigate 400D/500D etc.

I would like to know if i can implement transparent proxy feature with the above applaince for all my users including branch office without making major changes to network architecture

FYI, branch office are currenlty connected to HQ over IPsec site-to-site tunnel 

INternet outbreak is from my HQ office for all users including HQ 

Currently we are using explicit proxy using PAC file.

Appreciate all the inputs i can get which will help me with my evaluation.

many thanks in advance.

SV