Hi @eleanorw and welcome back to the Member Community.
It depends…on the type of SIM card you have! This will answer the question “Is It Okay to Swap SIM Cards Between Phones?”
There is a link in the document on “How to Tell If a Phone Is Active on GSM or CDMA”
New phones purchased directly from RW may come with a new SIM installed so all you have to do is turn it on and follow the prompts to activate the phone. Not sure what you mean by “guess” he should use new SIM. If the phone activated properly on the RW system it already has a SIM installed with no need to change anything.
If you will share your zip code, only the zip code, we can see if GSM or CDMA will provide the best coverage in your area.
Suspect you have a GSM SIM and moving it from the S7 to S10 will be fine. But the checks listed above will help confirm this.
Hope this helps!