I bought an iphone 4s 2 years ago on ebay ...fine. Activated it with Straight Talk byop. fine. Now I want to buy the same phone , unlocked, new. Can I simply take my present sim card , put it in the new phone ? Can it be that simple?
You need to make sure the new phone uses the same type of cellular signals/data as the old one, then yes thats all there is to it, besides transferring all your data over.
There were a couple of iPhone 4S models but not the huge variations we see like in the newer iPhone models:
iPhone 4S (Global) A1387
iPhone 4S (China) A1431
I'm going to assume your SIM Card is a Straight Talk (GSM-A) or (GSM-T) SIM Card and not a (CDMA) or (PCS) SIM Card? If so, it will work in either iPhone 4S model. (I've seen a few of the A1431 models in America that were purchased on eBay but almost all are the A1387.