It can be done, but it ain't easy. I switched from gen surgery after two years as a categorical, without ever applying to ortho before.
The best way of going about this (so I was told by those in the ortho world I talked to - mostly senior residents, staff, program directors, etc) is to do a year of ortho research and apply thru the match. This year will give you the opportunity to learn some ortho, establish a relationship with a program, get recommendation letters, etc. No program is interested in an outsider with no ortho experience, letters, etc. Trust me, I found this out the hard way.
There are programs out there with research positions that have a history of matching people out of their labs. This is the best option, in my opinion. It is important, obviously, to seek a place that will not use you as their lab geek for a year and then leave you hanging come match day. Unfortunately, I know of no program that will give you a guaranteed position in writing after the research year (except Wake Forest, and that is TWO years in the lab) because of the "Rules of the Match". Some places will even let you opt out of repeating the intern year, if you did a surgical internship and start after the research year as a pgy-2.
The bottom line is do your homework before you quit your current residency, get a research position on paper at a program that has a history of taking people out of their labs, and talk to everyone at that program you can from interns to the chair to see if it is a place that you can trust to come through come match day.
Obviously, you need to work your ass off in the lab as well. This kind of commitment is what program directors look for. They want to know you are serious. I quit my job with a wife and two kids to support, with no guarantee of a residency position on paper. There is definitely risk involved, but you can find a program that will come through for you.
No one should go through life doing something they don't want to do just because it may cost you an extra year or two to get where you want to be. If you want it bad enough, the risk is worth it.
Start by e-mailing all the program directors in places you would be interested in going to and just tell them your story and what you want to do and why. That's how I got started.
Good Luck