@Tim ********
based on what you’ve told.... your original plan would get you out of quarantine on the 21st, and your change would (potentially) get you out on either the 15th (best case scenario) in 20th (worst case scenario). Would this make a massive difference to you or your plans given the results? Not an answer or advice, just food for thought.
The actual answer to your question lies with the embassy and the Thai government. We can only give you (hopefully) productive ideas and options. As the rules and regulations have changed since you got issued with your COE no one here knows the answers for you, sorry. I do wish you the best though