How hard is it to chapter a Soldier with an approved retirement date?

If he is retiring, leave him alone. It used to aggravate me that ring bangers were so GI. He has done his time, why would anyone want to harass him now? Maybe he is old and worn out and cannot do PT like a 20 yoa soldier or a few too many beers have swollen his gut a bit, but that not a reason to throw him out short of his retirement

Since it likely means that Soldier has over 18 years of AFS (otherwise they wouldn't have an approved retirement date), you'll have an extremely high bar that has to be met because you'd be bumping up against federal law*. Because of this, HQDA must approve any involuntary discharge of a Soldier with 18 years or more of service unless it's due to the sentence of a court-martial or for physical disability.

This doesn't mean you can't process a Soldier to be discharged under another separation chapter, but it will likely have to be for serious misconduct (i.e., Chapter 10, 14, etc), and the separation will have to be approved at the Department level. If you were thinking along the lines of failure of the ABCP or something of the like, it likely would be a waste of time as the totality of service is considered when possibly denying an eligible Solider their retirement.

This doesn't mean a commander's hands are tied regarding other administration actions, if warranted, just that if anything affects the retirement it will have to be staffed through HRC to the approval authority (SECARMY or designated rep).
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* 10 U.S. Code § 1176 (retention of enlisted members with more than 18, but less than 20, years of service) - https://www.law.cornell.edu/uscode/text/10/1176
* https://www.hrc.army.mil/content/Enlisted%20Separations%20Process

MAJ Taylor, I think some of the comments to you from others are quite good. BUT...the Soldier's commander (if that's not you) doesn't have to guess about this. The commander's resource in this case is your command's Inspector General (IG).
My Brigade CSM and I would frequently use the IG to clarify policies and responsibilities prescribed in various regulations; even when we thought that we had the answer, or reasonably could understand the regulation. In this regard, Co and Bn Cmd teams should also employ the IG. Importantly, some regulations will state that the "commander MUST initiate a separation action" based on the deficiency/offense. In this case of a Soldier with an approved retirement, the IG can clarify the regulation and tell the commander what responsibilities and what options the Cdr has.
Save yourself time and headache and engage your IG and make a sound decision using good judgment.