I have just picked myself up a copy of "Only War" and was going to introduce some of the newer rules to my DH game, Such as the Single, Semi and Full Auto rules, Swift and Lightnining attack rules, introduce a Parry basic skill, maybe one or two others.

My question is for anybody who may have already done this, am I about to break the system with un intended consequences? As far as I can tell, these newer rules should not impact the game in any unforeseen ways, but just in case I have missed something.

Cheers in advance.

Sure! Take a look at my House Rules, link in the sig.

If you just port the combat rules over (and the like) there are practically no issues, no. It's a pretty seamless port.