Unlikely you'll be able to switch from an unpitched to a pitched percussion instrument mid-measure, if at all.
Your best bet is to put them on separate staves and hide what you do not want with Staff Styles and Hide Empty Staff commands. (You could probably move the staves to overlap one another where you wanted the mid-measure handoff if you are willing to work at it.
I'm just getting back to following the forums after the company move so my apologies for responding this late.
Since both crotales and glockenspiel are both pitched percussion instruments, switching mid-measure should be possible. This is not the case with non-pitched percussion because you need to set both percussion layout and midi maps.
My suggestion is to place each of the pitched instruments in separate layers. When you switch instruments in the middle of a measure, hide the rests for the other layer. For example enter notes for the glock in the first part of a measure but hide the rests in the remainder of the measure where the crotale part is displayed. For playback purposes, you will want to have each instrument loaded and use an instrument/midi channel change to switch the actual sounds. This may be done using a text expression which would also function as performance instructions for the performer.
Hi - Ted's post is the easiest way I know of also - using an expression to change the midi channel and patch.
If the layer trick isn't what you are looking for, this is fairly easy in a single line score - gets complicated in a full score... the idea that was shared with me some time ago is to create two measures that split at the beat point you need the part to change... hide the time signatures and then hide the shared bar line.
So in a 4/4 measure where you switch to Glock on count three... make two 2/4 measures - keep the first one crotales, then switch over to Glock in the second and additional measures. By hiding the time signatures and bar line, the viewer will see a 4/4 measure. In a full score, you would need to revise all measures in the stack - somewhat tedious... but for a single line staff, a fairly easy work around.
Also - I played around with assigning staff styles with changes mid-measure. It appears that it is possible to assign a staff style in a mid-measure scenario. I picked two styles from the default list and made a mid-measure selection - then assigned a single line staff - the image below shows how it turned out. I would be interested to know what worked the best for you.