I would purchase the "complete" 700 (for the price of the 500 in the USA - per your post) and then sort out exchanging the styli later as you already have the 500 body.
I'm frugal, but I also like backups @ a good price.
DeKay
Can a Nagaoka MP500 be converted to an MP700?
I am in Tokyo and considering whether to buy a new Nagaoka MP700 cartridge. Then it occurred to me that the stylus assembly of the MP500 is replaceable as a separate item. So I am wondering whether my MP500 can be effectively converted into an MP700 by simply replacing the JNP500 stylus with a JNP700 stylus. I do know that lesser models of the MP series can be upgraded by that approach. I am just wondering whether the resulting chimeric cartridge would exhibit all the virtues of an MP700. I realize not many of us are likely to have that information. But maybe.... Thanks for any response.