ESS/MSS Portal upgrade to 7.31+

We have a requirement to upgrade our existing ESS/MSS portal from it's current version of 7.02 SP11 to at least 7.31 (preferably higher, i.e. 7.4). Our back end ECC 6.0 system is on EHP 5 with NW 7.02 SP11. I've read the following blog, but it does not make any reference to NW stack of the back end system:

http://scn.sap.com/community/enterprise-portal/migration-and-upgrade/blog/2013/01/09/upgrading-to-nw-portal-73-what-to-do-with-essmss

My question is, can we upgrade the ESS/MSS portal (knowing that we would have to install ESS 633 on this new portal instead of the current ESS 603) without upgrading either NW stack or ECC of our back end system?

Thanks,

Setu