system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Do space marines keep their rank after they're put in dreadnoughts?

Could a captain or chapter master lead his marines as a dreadnought?
gpt
Part of the trouble with Dreadnoughts is that their entire existence becomes agony. Whatever is left of their shattered body is jammed into a cramped metal box, and all of its sensor and control systems are forced into their nervous system. Every second of consciousness is filled with pain and anger. This can, of course, be somewhat dealt with through the red-hot crucible of glorious combat, but it also explains the fact that when there isn't a battle for them to take part in, they are put into a sort of stasis. It is difficult for someone in stasis to maintain a Legion. 

Plus, once they are interred in their Dreadnought they are effectively immortal as long as the life-support systems function; Bjorn the Fell-handed fought alongside the Loyalists during the Horus Heresy, meaning this guy is 10,000 years old. And even though he's that old, chances are he has spent a vast majority of it in cold-storage. 

Bjorn is a good example of what you are asking, because he used to be the Chapter Master of the Space Wolves. Clearly, he is no longer the Chapter Master, but to call him anything less than one of the most revered and respected leaders and warriors in the Imperium's history is tantamount to heresy. 

I think the answer to your question is "kind of". Such is the reverence for these soldiers that they are almost placed outside of "rank". They'll still take orders from those "above" them, but few would ignore a Dreadnought's command. It is understood that they are the cream of the crop. These guys were so badass that when their bodies became broken and useless it was deemed more costly to simply let them die. Few are unable to recognize the significance of this, and as such, they are treated as being akin to their commanders. 

**Tl;Dr = They don't keep their rank, but they don't really need to because nobody is particularly willing to question them. They are seen as paragons of what is is to be Astartes, and so are afforded the reverence and respect of true commanders.**