How to add node on Upgraded OS

Experts,

Due to Storage Migration failure on 3 node RAC(10.2.0.4) on HP-UX IA 11.23(Autopath is not compatible to v400) , it is advised that upgrade the HP-UX to  11.31 for storage migration to V400.

So, Question is, is that possible to add a new node(HPUX 11.31)  to cluster running on old hp 11.23?

if so, what is the exact steps involved?

I was suggesting the business that instead of installing 10.2.0.4 binary in new OS and adding that to cluster and removing old nodes one by one, is upgrade DB/CRS from 10.2.0.4 to 11.2.0.3 feasible?

However, the concern was , this is 24x7 instance which wil be affected if we move entire version ,also it needs complete testing before move to production. That make sense, but am still thinking any better possibility to move to 11.2.0.3 without affecting the availability of instance, so that it will be available to customers and at the same time it wil help us to move to new version??