I believe the answer is "yes" and "no".
If I understand you correctly, you have one dish with a Pansat PF-7600 connected to your DSR-315. If I'm wrong, and you have two dishes w/ PF-7600's, the answer is "yes" and please ignore the rest of this post.
Assuming you have only one dish/LNB, you must be currently pointed at Anik F1R or Anik F2, since the antenna setup you have is capable of receiving from only one satellite at a time.
If you upgrade to a DSR-605, I'm presuming you are now interested in adding HDef programming. You will continue to get whatever SDef programming you now receive, plus whatever HDef is on the satellite to which you are aimed. However, to take full advantage of all the HDef channels in the lineup and to retain the SDef channels you currently receive, you need to be able to see both Anik F1R and Anik F2 simultaneously. Note that most of the HDef stuff comes from Anik F2.
So I believe your options are to purchase a second antenna setup similar to what you already have (and connect them with a switch) or to purchase the standard Shaw Direct 75e dish with the quad LNB.