SAP BW 7.0 to 7.5 upgrade Infozone and ECC netwearver at 7.0

Hello Experts,

Good Evening.

We are upgrading SAP BW from 7.0 to 7.5 . i.e BI 7.0 SP34 to BI 7.5 with latest SP's on non-HANA DB (MS SQL).

We are planning to upgrade our BW systems separately independent of ERP and portal system which are connect to it.

Current version of ERP and Portal systems are as below:

ECC: NW 7.0 ECC6.0 SP24
Portal: NW 7.0 SR3

Here is the question, can we upgrade and go live the BW system without upgrading other systems i.e., ECC and portal.

If so, what are the implications/risks appears and if any pre-requisites to taken care.

Please help.