Blue pill or red pill = 675 or 920

If your existing heads are working fine, leave them alone and drive it. If they need changing, I'd just install the 675's and go. I'd wait until I wanted to do a complete build to change to the 920's It may be that installing them will boost the compression ratio up to where you have to run high octane fuel. You don't really know until you compute the actual compression ratio during a build.