Nice bike.
Tektro 539's will get you to 700c wheels, but no larger than 30mm tires as mounted...without fenders.
For 650b, you'll need Tektro 559's, and likely better brake levers. Limit there is likely 42's. Again, no fenders.
Either way you'll get into HG freehubs, which is good. And brake pad holders that can accept upscale pads, also good.
I like the sweet spot of 700 X 28, maybe Conti GP's. Match the 539's with some Tektro levers. Good ride, durable, flat resistant, fast...and you can run fenders with them.
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