I may need to replace my 5 year old Shimano 105 RD. I have a 9-speed cassette and brifter. I sort of remember hearing that I could just use a new 10-speed 105 RD to replace the older RD. Is this true? Or do I need to find a 9-speed RD to replace my current 9-speed RD???

I'm not trying to change to 10-speed. I'd use the new RD on my 9-speed cassette with my 9-speed brifter.