That's what I thought: Just change the SIM, tell everyone of your new nmber, and you are pretty much ready to go.
Is it possible that changing the SIM would affect her NON-google app data, like whatsapp, Naver Line, viber, and facebook??? Is that what the store who was using the sim card might have been trying to tell her?
I don't use it personally, but I believe Whatsapp creates a messaging account based on your phone number, right? Don't know what the app would do if the phone number changed.
EDIT: Actually I just had a look over at their FAQ and it seems that they will have you
verify the new phone number. But it won't migrate over your old account info
unless you follow this process, which seems like it would be inconvenient to do every single time you swap SIMs.
And actually the first link advises not to swap SIMs too often as it can cause problems with verification.
But for all other apps it should be fine as as long as they are linked to the Google account or locally stored and are not reliant on phone number like Whatsapp. But I would double check just to make sure.
I happened to remember this detail about Whatsapp because some relatives tried to drag me into using it a while back.
EDIT2: Did a bit more googling, it looks like
you can also leave Whatsapp configured as is and people can just message you at your old (original) number. Apparently the only downside this has is if you lose your original SIM card and lose your phone as well, there will be no way to retrieve your Whatsapp account and you will have to create a new one.
Or at least that's what the internet said. I take no responsibility for the completeness or accuracy of this information
but you should be able to verify with a bit of your own research