i did this and kept the same stem, it was fine for me,
my position went from bullhorns - streched out on the bike, with a flatter back
to moustache, on the same stem, just meant my hands were back by about 20cm probably, with a 10cm stem, my hands were at the same height - but inline with the headset pretty much.
i always think its good to change one part at a time, then if you want a shorter stem as well... go for it, although this exaggerates the action of the moustache bars by bringing them even closer.
i did this and kept the same stem, it was fine for me,
my position went from bullhorns - streched out on the bike, with a flatter back
to moustache, on the same stem, just meant my hands were back by about 20cm probably, with a 10cm stem, my hands were at the same height - but inline with the headset pretty much.
i always think its good to change one part at a time, then if you want a shorter stem as well... go for it, although this exaggerates the action of the moustache bars by bringing them even closer.