How can I reclassify from an overstrength MOS to an understrength MOS before my window opens in 2024?

You're mixing up your reenlistment window with Reclass. As long as you've been on station for a year, you can request to reclass from an over strength to an under strength MOS. It's a pretty simple process, the hard part is class seat availability.
Just go see your Career Counselor

You can submit a voluntary reclassification request outside of your reenlistment opportunity window (ROW) for an under-strength MOS. Below is the guidance for a voluntary reclass:

(a) The Soldier’s current PMOS must be listed as overstrength (N/Y) and the requested MOS must be listed as short (Y/N) on the current Reenlistment and/or Reclassification IN/OUT call message published by the HRC.
(b) Soldier must meet the required criteria of DA Pam 611–21 for new MOS.
(c) Soldier cannot be on AI. (Soldier is considered on AI when assignment has been processed through EDAS, not date of notification.)
(d) Soldier must have completed 12 months at their current duty station.
(e) Soldier may not be serving on initial enlistment under the Army Civilian Acquired Skills Program (ACASP) (see AR 601–210).
(f) Soldier must agree, in writing, to recoupment of any unearned portion of the MOS specific enlistment and/or reenlistment bonus received for the current period of service (see AR 601–280).
(g) Soldier must be within 12 months of the DEROS.
(h) Soldier may not be within 24 months of ETS.
(i) Soldier must have fulfilled all SRRs for promotion and assignments.
(j) When stabilized because of reenlistment, Soldier must waive their reenlistment option.
(k) Soldier must be fully eligible to reenlist or extend per AR 601–280.

Another option for reclassing outside of your ROW is to submit for an Application MOS. These MOS's require various feeder MOS's, GT Scores, rank requirements, selection processes, etc. You can apply for these at anytime.

CMF 18 - Speal Forces
12D - U.S. Army Diver
12P - Prime Power Production Specialist
17C - Cyber Operations Specialist
17E - Electronic Warfare Specialist
31D - CID special agents
35L - Counterintelligence Agent
37F - Psychological Operations Specialist
38B - Civil Affairs Specialist
51C - Contracting NCO
79S - Career Counselor
89D - EOD Specialists

Your Career Counselor is the best POC to process an application or submit a voluntary reclassification request.

SFC (Join to see) can provide you information, but you can only request to reclass during your reenlistment window