how to change hardware in a DAG setup

Hi,

I have a DAG setup which has 4 identical HP servers : 2 at the local site and 2 at the DR site.

The company has grown very quickly and of course i underspec'd the servers ......

So my question is : 

If i need to add bigger HD's to all servers , how does the process work in terms of hardware and also what is the process that i will need to do.

I.E : if i change HD at the local site, will it break the DR site? do i have to do all servers at the same time? is there a more streamlined way

thanks