[ssg]

I wish to change my bull's rear tyre 3.25 x 19 with classic's 110/90 -18.

What are the pros and cons?

Thanks in advance.

[Maturin]

The first con would be the cost, ad you'd need a new rear wheel or to rebuild a 18' rim around your old hub. Another one is the alteration of your bike's frame geometry: the new 18' lowers the bike's rear and prolongs the trail. As nobody tested this configuration before you'll be the first to discover any weaknesses.
Your stock rim is probably good enough to mount a 3.50-19, as this is stock with the 500ccm-version. I'd recommend not to mount anything bigger. I actually don't see any advantage in terms of riding dynamics, you possibly do have more brands to choose from with a 18'-wheel. But if you take the cost for the modification into account, you could import a decent Dunlop K70, K81 or Heidenau K34 and still save money.
Best regards