It is really hard to say how an ID change may affect eBay search visibility in Best Match. What you did in changing the eBay ID name is something that is not often done.
To me it should not matter as it should be transparent. Anyone watching before the eBay name change should still be watching.
As for eBay customer support, they often know little about how things regular work on eBay and give some poor advice. For something as rare as an eBay ID name change, I really doubt they know much about that.
I would suggest waiting it out a few days more. If you start messing with other parts of your listings or the way you list, you may make other issues. Patience. A few days to see if things go back to normal. What was working before should still work after the eBay ID change.
For all sellers, there highs and lows of sales. You may have hit a natural low that by coincidence happened with the eBay ID name change. How is your seller dashboard in My eBay -> Account -> Seller Dashboard? Any recent low DSRs?
On second thought, just wait it out for a few more days without trying to figure it out. Too stressful. Assume it is a natural low and wait it out. Then start to look into reasons why.
