Is this statement correct for how Transit buses are upgraded?

This is the statement I am referring to:

"Transit busses have models that are upgraded to new styles every so often, not by model year. If your city council or board of governors wants to upgrade their transit busses, they will see several models with varying features… Low floor design, air conditioning, fully automatic 6 speed transmissions that will give maximum economy at all speeds, length, capacity, and so on."

I know cars (such as the Ford Focus, Toyota Camry, etc) get upgraded by model year, but do Transit buses have models that are upgraded by new styles every so often, not by model year?
a) yes
b) no

When you answer, can you give me an explanation?

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