Bike Forums - 9 & 10 Speed Interchangeability..?

I'd say "yup" with the possible difficulty of an 11 tooth gear on an older hub. From the late great Master Brown:
http://sheldonbrown.com/speeds.html

Shimano Freehub Cassette Systems (and copies)
In general, all Shimano Cassette Freehubs will work with all Shimano cassettes, any number of speeds. There are a very few exceptions:

7-speed hubs won't normally accept 8-, 9- or 10 speed cassettes without modification. Click here for details.
7-speed cassettes fit fine on 8- and 9-speed (and most 10-speed) hubs if you put a 4.5 mm spacer onto the body before the cassette. Click here for details.
Older "Uniglide" Freehubs (including all 6-speed units) won't accept modern cassettes without modification. The last Uniglide Freehubs were made in the 1989 model year. Click here for details.

These older hubs can usually be upgraded to work with modern Hyperglide cassettes by replacing the Freehub body. Click here for details.
Some Freehubs are not designed to accommodate cassettes that include an 11 tooth sprocket. Click here for details.
2004-2007 Dura-Ace hubs with aluminum Freehub bodies won't work with anything but 10-speed cassettes. Click here for details.
Capreo hubs and cassettes (with the 9-tooth smallest sprocket) only work together. Click here for details.

Sorry about the nonfunctioning links, but you can see the original site. As to th e11 speed on an older hub problem:
http://sheldonbrown.com/k7.html#hyperdrivec
"Hyperglide-C" cassettes on conventional bodies.

A wobbly cassette may result from a loose lockring, or wear of an early Uniglide hub, but there is another potential cause.

Shimano uses the trademark "Hyperglide-C" to designate a system with an 11 tooth sprocket. The "C" stands for "compact". These systems are used with smaller-than-usual chainwheel sizes, or on bicycles that have a small drive wheel, or to achieve higher gears.

Due to clearance problems, the cutaway between the splines on 11-tooth sprockets only goes halfway through the sprocket. The matching splines on Hyperglide-C bodies don't go all the way to the outer end of the body.
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Hyperglide "C" body Non-compact body
If you install a cassette with an 11 tooth sprocket on an older, non-compact body, the cassette will not be properly secured, and the sprockets (other than the 11) will be loose and wobbly.

There are two ways around this problem:

You can use Hyperglide-C cassettes on conventional bodies by adding a 1 mm thick spacer to the body before installing the cassette. This is a standard spacer commonly used for fine-tuning chainline with conventional freewheels. It may be necessary to add a spacer to the right side of the axle in some applications, especially if you wish to make the wheel interchange with other wheels without needing to re-adjust the rear derailer.
Alternately, you can grind a bevel on the ends of the splines of an older body. This is a five-minute job if you have access to a bench grinder. Clean up carefully afterward so particles of metal and grinding-wheel grit don't wear out the bearings.

Note, if you install an 11-tooth sprocket on an existing cassette that had a larger top gear, you must also replace the Hyperglide lockring with a Hyperglide-C lockring. The lockrings made to work with 11-tooth sprockets have a smaller outside diameter. If you use a larger lockring, the side plates of the chain will hit the edge of the lockring, and the chain will not run properly on the 11-tooth sprocket. (Hyperglide-C lockrings are compatible with 12-tooth sprockets though not always with larger ones.)