Yah it was Isaac.
Working around AC wiring can be extremely dangerous so be very careful! To discharge the caps safely, go buy one of these:
https://amptechtools.powweb.com/stick.htmor make yourself up something similar.
Also, I'd suggest to buy this book:
http://www.tubesandmore.com/scripts/foxweb.dll/moreinfo@d:/dfs/elevclients/cemirror/ELEVATOR.FXP?item=B-732This book is rather basic for tube amps, but has great instructions in the basics that most can understand. It also has full page schematics and layouts of Fender amps which are very useful even if you don't own any Fender amps.
To change over to a 3 wire cord, there is a bit of work involved and its very important that you get it right so that it will be secure and not short to anything inside the amp. The 2 wire practice was to put the black (hot) line on one side of the AC, the white (neutral) on the other, and have the fuse and switch on either side of the AC, with a switch to switch the ground connection to whichever end is quieter. To change to a 3 wire setup, a green wire gets added and this goes directly to the chassis on it's own lug within an inch or two from the AC plug on the chassis. It's easiest to use the IEC plug but you can use the wire and grommet method too. The black gets wired from the wall plug to a fuse, and then to a switch and then to one leg of the power transformer. The white gets wired to the other leg of the transformer. You may have to add insulated and isolated terminal strips so you can solder the connections of wires together. If you have things wrong, then it can blow your power transformer btw. Best to do any testing after you think you are done with no tubes in the amp. Also best to draw up a layout of how it is now, and draw up another one of the changes you think it will need. If nothing else it will help you to visualize what is needed. If any of this sounds hard to you, then please do the safe thing and take it to a tech, or find someone who is knowledgeable locally who can help you, or learn the proper way to do it before you tackle this type of job. I read everything I could find, asked questions of friends who are electrical engineers and spent 4 years reading and reading and looking at amps until I was sure I knew what I was doing. This is because when I first started I shocked myself while messing around on something and knocked myself across the room. I was lucky I wasn't killed, and felt like I had beat up for weeks afterwards. Now I am extra safe any time I touch an amp.
Greg