Thanks to mAx___ at the Vintage Amps Forum for sharing know-how and experience on upgrading the output transformer and choke on the AC30HW. The original thread also deals with upgrading the filter caps and can be found here.
This is the process I used to replace the OT and Choke:
1) Remove the back cover and bottom of the chassis.
2) Cut the nylon ties that secure the primary leads to the chassis. Take out all the screws of the turret board and lift it till you find the three cables in the picture. The OT has six leads that need to be unsoldered to remove the existing transformer. The choke has only two. The red one in the pict comes from the primary side of the OT and the black and white from the choke. Unsolder all three. Remove the choke.
6) Bundle the primary leads (red, yellow and white/yellow) and pass the red one underneath the board like in the picture (see left picture). Solder it to the same turret that the old red lead was attached to in step 1). The picture shows the bundled cables secured to the chassis with nylon ties. Leave that alone for now, it should be the final step after testing that everything works as intended.
8 ) Solder the thick yellow 16 ohm lead to the top position on the impedance switch and the thin green 8 ohm lead to the bottom. Attach the black wire to the ground screw and tighten it.
9) The choke will need an extra hole drilled to the chassis. A possible solution to avoid this would be using one of the ventilation holes already in the chassis. Attach the choke with the bottom screw first and bolt the top to one of those holes. I didn’t try this but it looks like it should work fine.
Both leads in the MM choke are black and I understand that it makes no difference which one goes where. Solder them to the bottom of the board in the two positions where the originals were attached.
That should be it. Secure the board with a couple of screws for testing and if it works put everything back together and enjoy the amp. I hope I made sense and that I didn’t forget any important details. Please comment on any possible mistakes or better ways to do it!