Just need confirmation for this. I think the answer is yes, but I am not sure.
Is spacing indexing really the same for shimano 7, 8, and 9 speed derailleurs for the last 7 speeds? 9 speed cassette + wheel available, but my bike only has a 7 speed Shimano exage motion derailleur on it.

A 7-speed derailer will shift over a 9-speed cassette just fine.
The indexing is done by the shifter.
But you do need to use a 9-speed shifter - or friction shifting - to be able to use a 7-speed derailer on a 9-speed cassette.
You can’t put a 9-speed wheel in an otherwise 7-speed bicycle and get 7 working gears.
You can put a 7-speed cassette on a 9-speed wheel by adding a 4.5mm thick spacer.

I use my ancient Deore Deerhead rear derailleur with a 9-speed cassette with no problem. I can run it with index or friction shifting.

Cheers

So there is a spacing difference between cassette hence the need for specific shifters?

Is the shifter 9 speed or less? Did you have to tune the derailleur at all or was it right off the bat?

Yes. A 9-speed cassette is about one speed bigger physically despite having two more sprockets.
8-of-9 is a sometimes used shortcut among those wanting to use a 9-speed shifter and cassette on a 7-speed wheel.

When I used it in index mode it was with a Dura Ace 9-speed bar end shifter. I just set it up like you do with a normal 7-speed or even 5-speed system.

Before connecting the rear derailleur cable I adjusted the high and the low limit bolts on the derailleur and checked to motion and range of the derailleur. Then I added the chain and the derailleur cable. Easy peasy.

Cheers

Hopefully I have interpreted the OP correctly. Conversion from a 7 speed to a 9 speed wheel/cassette will require more than just putting the wheel in place.

1. The space between cogs in the cassette becomes smaller as the number of cogs/speeds increases. 7 speed spacing does not equal 9 speed.

2. The indexed shifter controls the amount the derailleur moves with each detent/click/speed. You must use a 9 speed shifter with a 9 speed cassette. Much of the time, the brand of shifter and the brand of derailleur must be compatible. Many brands such as microshift and sunrace are built to be compatible with Shimano. Suntour, SRAM and Campagnolo mostly don't work with shimano derailleurs. An inexpensive alternative would be to use a friction shifter but if you have never used friction you may find it frustrating.

3. The 9 speed wheel is built to fit in drop outs that are spaced 130(road) or 135(mountain) mm apart. This is called the OLD (Over Locknut Dimension). The OLD of your bike could be 126 mm. If the frame is steel the drop outs could be spread (cold set) to the wider dimension. If the frame is aluminum or carbon fiber you are out of luck.

4. Many people use derailleurs originally sold in fewer speed drivetrains with 9 speed cassettes and shifters. I have used a 7 speed shimano derailleur with a 9 speed sunrace shifter and cassette and it worked perfectly. That does not say that your current 7 speed derailleur will work with the specific 9 speed cassette you have. Your derailleur may not have the capacity to accommodate the largest cog of the cassette. It also may not have the take up capacity to handle the difference between the small-small and big-big combinations of the new drive train. Capacities would only be a concern if the new cassette has much larger cogs. For example, if the 7 speed cassette is an 11-28 and the 9 speed is an 11-34.

5. The outside width of the chain becomes less as the cassette spacing decreases. A thicker 7 speed chain will not work with a narrower 9 speed cassette.

6. SheldonBrown.com is a terrific source for all your questions. You may want to study it a little before proceeding.

It appears to me that if you go ahead with the 9 speed wheel/cassette you will probably need to cold set the frame, obtain a nine speed rear shifter (shimano, microshift, sunrace) and obtain a 9 speed chain. You could also need a different derailleur. After assembly, adjustments will be needed to make it work correctly.

I hope this is not too confusing and helps a little. Remember SheldonBrown.com. Good luck with the project.

I see now. I had thought as the the cassette disk count increases, the freehub length increases, hub gets shorter, and the disk spacing remains the same, but that's not the case. Not as backward compatible as I had hoped. Back to drawing board.

That has happened too. A ”true” 7-speed fh body is shorter than an 8+ speed body.
As speeds and number of sprockets are increased, the sprockets become thinner, the gaps between sprockets become narrower.

There are several workarounds for mixing different versions of drivetrain parts. Tell us what you want to achieve and maybe we can suggest something.


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OP. You can use a 7-speed derailleur to shift a 9-speed cassette.

If you have a 7-speed cassette and want to use a 9-speed cassette on the SAME hub you can use iirc 8 of the 9 cogs. and leave off the smallest cog or the biggest cog depending on which gear you need more a high gear or a low gear.

BUT

To use 9-speed cogs you'll need both, 9-speed shifters and a 9-speed chain.

Cheers