IPhone 6s Plus got stolen a few nights ago?

Using my Find My iPhone app, it says its been offline since 9:01 am today. It is now 5:36 pm. If the person takes out the sim card and replaces it with a new one, will that make the iPhone useable? I put it on Lock mode too.

Depends on what their provider is

If the person takes out the SIM card, it won't be able to connect to the carrier, thus it won't have mobile Internet. Without mobile internet, the iPhone can't receive the Lock mode that you sent it.

However, once the person connects the phone to Wi-Fi, or puts in a valid SIM card, it will be put into lock mode, and you should be able to find it once more.

Look like? Look like?

Well, they can reset the iphone or brute force and use it.

As long as you put it in lock mode they can't ever use it, it will notify you when connected. You can also put a msg on screen saying stolen, they can't remove that either.

Don't believe the ones saying yes. They haven't got a clue. You can change sim cards all day, or reset, or do anything else you wish to try, but that won't allow a stolen iPhone to connect to any phone service.

Each time anyone attempts to connect to a phone provider, the iPhone sends its internal ID (NOT found on the card, but in the built in ROM and NOT changeable) which has to be verified in the system. If it is not correct, the service will NOT connect. It will also cause the service to "flag" the info on the sim card as using a stolen iPhone. That's what the blacklisting is all about for lost/stolen iPhones.

People are beginning to be visited by police when they attempt to use their sim card in their "new" iPhone, which happens to have been stolen somewhere along the way. The phone company has the address of the person who owns the sim card, so it is a simple matter to forward that info to law enforcement.

Remember that if they arrive before a person turns it in after discovering that it is stolen, (if it won't connect, it is stolen. There's no other reason!) that person is in possession of stolen property (which is the same penalty as actually being the thief) and can be arrested on the spot.

If it is not located, it means that someone has not turned it on. It probably will be found on Craigslist, or eBay in a while, being sold to someone who will think they got a "great" deal (many of them come to "Answers" to ask for help when they discover what they actually bought!) but end up being the next one who can't connect. It could go through many hands before someone finally takes it into a phone store and tries to sign it up and gets arrested when it shows up as stolen, or actually decide to do the responsible thing and turn it in to the police, phone company, or Apple store. (No one will file charges against someone who tries to turn it in. They will file charges against someone in possession, who has NOT turned it in!)