I'm sure I'll get some backlash on here, but Disney IT is rough (and it really makesno difference at the end of the day)...if it were me and she wasn't the lead guest or anything; I'd just hand your mom your sisters magicband or grab a card thing from the resort when you arrive pretending she's your sister...it's just sooo much easier. I actually did this when I changed out a sister 2 weeks ago.
If you insist on changing it to your mom; I would do NOTHING over the phone and instead go to guest relations as soon as you get there and explain the situation...they'll be able to fix everything...it'll just take some time...the other way wastes no time, but I know all have their preferences.