All intentional radiators must be certified to the FCC regulations. Since you said "transceiver", it implies this device is in part a transmitter. Selling a intentional radiator, such as your device, without FCC certification in the United States is a federal offense. All your units can be confiscated, you can be fined, and in some cases worse punishments may be imposed.
Frankly, if you are asking such a basic question, you don't belong in this position. Get someone who knows what they are doing to help you thru the process. Then maybe next time you can be the expert. This really not a place newbies belong without the guidance of someone that does know what they are doing.
Absolutely it is required, I assume you are talking about being a radio in the 915Mhz band? In which case you will be an intentional radiator and will need to go through compliance test at an actual test location, and register the device to get an FCC ID, which you must then apply to your product.
The specific section you are interested in is FCC section 15.249, here is some more information about it from the FCC.
Here are some good app notes, one from TI the other from Semtech.
You'll see that there are also power limits for simple transmission, and higher power limits if you're using spread spectrum.
You will also of course have to pass FCC Class A or B depending on your equipment type.
Edited to answer questions
I'm no FCC or legal expert and you should probably consult one or more of those and a test house. However if you look here you will see the FCC guidelines to "Determine if device requires authorization under Verification; Declaration of Conformity (DoC); or Certification" Where they say for "Unlicensed transmitters (low power intentional radiators) – see Section 15.201"
Paragraph 2 of that section says
"Except as otherwise exempted in
paragraph (c) of this section and in
§15.23 of this part, all intentional radiators
operating under the provisions of
this part shall be certificated by the
Commission pursuant to the procedures
in subpart J of part 2 of this
chapter prior to marketing"
Now subpart J of part 2 states:
"§ 2.907 Certification.
(a) Certification is an equipment authorization
issued by the Commission,
based on representations and test data
submitted by the applicant."
That's your section saying you must apply and provide data for certification, you can read subpart J to learn more about how that testing should be done. I suppose you could collect the data yourself provided you have access to the tools (and shack or are in a valley somewhere with no interference), and that you can follow an acceptable test procedure. A little more digging will show how you have to apply along with the fees.
Or you could go to a test house and pay them, which is what most people do.
Best Answer
FCC testing usually includes rf testing of the unit in the case, unless it is a certified modular device. Changing the case will likely require retesting, and certification.
Keep in mind that even generic device's that are rebranded, only cosmetically, will often have their own fcc id. That's for devices that don't have case changes too.
Of course we are not lawyers, and the fcc process comes with heavy legal implications. Consult a lawyer. They may just say to recertify as a precaution. An upfront potentially unnecessary cost is easier to deal with than a large unforeseen fine or recall.