If you are now under the Simple Delivery process, and choose to use your own postage and not that from SD, you will not be able to change the tracking details, and will now need to wait 14 days for your payout.
You could try and ask Customer Services if they will change the details for you, no harm asking, but in my humble opinion, g if eBay are now advocating SD for Private sellers, they may well not oblige.
If you wish to, this is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
I recommend contacting CS first thing in the morning as there's more chance of Dublin answering.
@woodview125