Hello,
I am trying to switch the vertical stacking order of the upbow
symbol and the trillspan in the following fragment by overriding
the script-priority of the upbow symbol.
The upbow symbol should be the topmost.
\version "2.22.1"
\relative a'' {
g1\startTrillSpan ( ~
\once \override Script.script-priority = #1000
g2\upbow\stopTrillSpan ~ \acciaccatura fis8 g8 e8)
}

I tried replacing "Script" with "TextScript", but it did not work
either.
And I tried overriding TrillSpanner.script-priority to -1000
(placing it before the g1), which also didn't work.
Can someone please be so kind to explain how to accomplish this?
Regards,
Arjen
Try using outside-staff-priority instead. Also, I moved your \stopTrillSpanner over so it extends underneath the upbow.
\version "2.22.1"
\relative a'' {
g1\startTrillSpan ( ~
\once \override Script.outside-staff-priority = #1000
g2\upbow ~ \acciaccatura fis8\stopTrillSpan g8 e8)
}
Someone else will have to explain why script-priority hasn't any effect. That would enlighten both of us.
Hope This Helps!
Mark