I’ve done it before on my own bikes and second hand wheels with no problem.
I’ve just been asked to do it on a brand new Specialized Myka with “Alex HR 26″, alloy double wall” rims.

It’s just the thought of making changes to a brand new component that’s still under warranty that concerns me.
I’ve assured her it’s safe. Is it ?

go for it’s not your bike!!

If it fails tell her to tell the LBS Graham said it was OK 😉

if they break and manuf notices the holes I’m sure they won’t warranty, regardless of any involvement of the holes, but drilling is fine IMO/E (me not being an engineer etc)

why not just use presta tubes?

Because there’s a woman involved and that is too easy.

I used to build a lot of wheels and I’ve drilled out hundreds of rims to take schrader valves and there is no reason why you shouldn’t go ahead and do it.

8mm drill bit works great.

snap a presta valve(easily done) on a cold winter night ride miles from the van with a repair kit but no spare tube and you will curse the day you never drilled and swapped,,,,,,,,,,,,,, i did anyway 8O.

always drill them never any problems, almost split a rim at the pins once but the drilled hole was fine.

cant see the point of fancy italian road bike valves on a MTB

^^^^^^ carry spares is the answer for your issue not drilling rims
t will be fine – i have done loads including some Alex ones

OK, I’ll drill them then.
I’ll be converting it to tubeless at the same time. 20″ Shrader tubes are far easier to get hold of.