How do I solve the issue where the grand piano no longer sounds like a piano?

Have you compared the settings for the "Grand Piano" track, between your laptop and your iMac? You may have accidentally saved changes to "Grand Piano" in the "Edit" panel for the track in the "Info" panel. Check, if the "Sound Generator" is still set to a piano sound, and if it is the same as on your laptop, and check, if any effects have been added.

This looks like a part of your instrument library may missing.

What are the MacOS X versions on your laptop and your iMac? Is your profile signature still correct?

imac, Mac OS X (10.6.7), Intel Core Duo

Did you migrate your system from the laptop to your iMac, when you setup your iMac? And use Migration assistant? Then the jampack instruments in the instrument library may not have been copied over, as Christoph Drösser reported (Re: Why are my Jam Pack voices not showing in GB?). If that should be the case, compare the Intrument libraries on both machines and copy the complete Instrument library from your notebook to your iMac, if instruments should be missing.

In MacOS X 10.8.5 the instruments Library is installed in

/Library/Application Support/GarageBand/Instrument Library/

I don't remember, if it was in the same location in MacOS X 10.6.x.

-- Léonie

Did you reboot the system, after installing the instruments?

And do you now see the same instrument menu on both your macs in the "edit" panel for your piano track? If yes, after setting the sound generator to "piano", have you checked, if you are now using the "grand piano" or the "Steinway piano"? Like in my screenshot?

Have you checked in the Trackhead, if any automation curves have been added to your piano track?

Look again into your Instrument Library.

In the folder

/Library/Application Support/GarageBand/Instrument Library/Plug-In Settings/Piano/

you should find the following instruments:

and you should have plenty of wav files in

/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Files/Grand Piano/

also Grand Piano files in

/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Instruments/Grand Piano On Stage.exs

Is that what you are seeing?

I get this, when I move the file

/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Instruments/Grand Piano.exs

to the Trash.

Check, if this file is missing or corrupted in any way. Compare the file sizes between your two macs. Maybe the protections are not the same. Compare, what you you see in the "File > Get Info" panel.

Then I really have no more ideas. BTW, when I remove the "Grand Piano.exs", my piano sounds more like a horn or organ, not like an instrument with strings.

Have you installed any additional audio components on your iMac that could be interfering? Any Midi drivers, plug-ins? Check this folder for Components and plug-ins:

/Library/Audio/Plug-Ins/

I was thinking of uninstalling garageband and then reinstalling it. Would this solve the problem? or should I run the upgrade again? is that even possible?

What is your current MacOS X version? How to uninstall will depend on that.