My dad won't be needing his 2 year old iphone SE anymore.

I'd like to set it up for mum to use.

She currently has an old Samsung flip non-smart phone.

I expect I'll have to get a smaller sim card for her phone number.

Once I've got that, is it just a simple matter of taking dad's sim out and putting hers in?

Will the PIN and contacts etc remain? 

I have considered getting mum to just assume ownership of dad's number and relinquishing her number but there a couple of complications with that, namely getting phone calls for dad, and from people she may not wish to talk to.

Any other things I should consider?