I went the other way on this, replacing a Suntour 5 speed cogset with an Ultegra 9 speed cassette. Using a new wheel, of course. But my old front chainrings work fine with the 9 speed chain and friction shifters.

So I guess I did the exact opposite with no problems, I can't imagine you'd have any problems doing it the other way as long as you stick with friction shifters.

Az