Can I buy an IPhone 5s and then switch from my ipone 4s to the 5s without an upgrade for verizon?

Since verizon dropped the 5s to $99 can I just buy that and then switch my 4s to a 5s without an upgrade?

No, the $99 price is "upgrade price" - you have to be eligible for an upgrade / contract renewal.
If you don't have an upgrade available, it costs $649.

What exactly do you mean? Are you saying that you have an iPhone 4S and you want to buy the iPhone 5S because Verizon just dropped the price to $99?

When you have Verizon you have a 2 year contract with them. When did you buy your iPhone 4S? Was it two or more years ago? If it wasn't, then that iPhone 5S will cost $649 since you're not out of contract. It will only cost you $99 if you're out of your two year contract or if you bought your iPhone 4S contract free.