You will need to change rectifiers, and probably some components in the cat bias circuit. That would be the easy part.  If you email Nik at Ceriatone he will get back to you real fast. He can advise you about the necessity of changing your Output Transformer, and any other details.
Good Luck

What are you running for speakers?  Switching to more sensitive speakers can be the equivalent of a several watt boost.   Or, hook up a 1x12 or 2x12 closed back cab and that little champ will be plenty loud for small gigs.

IIRC, (and that's a big 'if'), Niks PT can easily supply enough heater current for a 6L6 or KT66.

To get the most out of those tube though, you'll want to increase B+ voltage by using at least a 5AR4 rectifier or even a solid-state rectifier plug.  A 6L6 will run cold with the stock 5Y3.  An increase in B+ voltage should not only get out output from the tubes but also get the cathode bias back in range.  Though I would always take measurements and check dissipation anyway to find the optimum range.

At this point the limiting factors are the OT, which may be too small choke off many of you new found watts.

And you have to do the math for the impedance.  The Ultra Champ being multi-tap, that's relatively painless.  If running an 8 ohm speaker, you will change the impedance switch to "16 ohms."  With the swap to the 6L6, you primary impedance is nearly halved, and so the secondary will also be about 1/2 of what the label says.