I would advise you not to install firmware on your KN2600. Usually the firmware updates don't add important features, most usually they correct spelling errors in various languages, stuff like that. It won't give you new styles or sounds.
Installing firmware can damage your keyboard if it goes wrong and there's nobody who can help you if that happens.
You say you have an SD card with KN7000 styles that works. This makes me think that perhaps you aren't transferring styles to your other SD cards correctly.
To transfer styles to a card you need KN-SDExplorer or other similar software. You can get the free demo here:https://software.technicskn7000.com/
Instructions are also on the site, read the sections carefully.
You don't need to buy the full version of KN-SDExplorer, the free version will transfer 10 styles. Then you can re-start it to transfer another ten and so on. Or maybe you have it already. If you bought it in the past I can arrange for the license to be sent to you again. If so, send me the email address you used at the time AND your current email address via a private message and I'll organise for the license to be sent to you.
Remember that you must format the SD Card in your KN2600 and you must make a directory (using the keyboard) and save something onto the card also (using the keyboard), to prepare it correctly. You can save anything onto the card. After that, you use KN-SDExplorer to transfer styles onto the card.
This procedure is exactly the same for the KN7000. There is no difference in how the KN7000 and KN2600 work with SD Cards but you don't have the benefit of being able to transfer from floppy to SD using the KN2600 so you have to use KN-SDExplorer or similar software.
Also, if you have a KN7000.sdb backup on your computer, the KN-SDExplorer software will restore that backup to a blank card for you.
I am still working with the programmer to bring out a new version of KN-SDExplorer but it is very time-consuming and we've both been delayed due to work and illness. However, the current version works fine in all versions of Windows which is testament to the original programming. Just that we are (still) working on a simpler version, it is not forgotten.
Some 2Gb SD Cards may work but the officially supported maximum size is an SD-Secure Card of 1Gb. SDHC and other types of SD cards won't work at all.
Over the last couple of years I've had some problems with my sites but they now run on very fast servers and most of the problems are resolved. I'm constantly working to improve them when family time, work and health allow and I'm also very grateful to John (JD5live) and Roger (Rmepstead) and Dave Tutt for the help they give me and the members because I simply can't cope on my own.
John and I own most models of Technics keyboards between us and we have a KN2600 so I can assure you that the above information is correct and we'll try to help you out where we can when time allows but it does take a bit of learning on how to transfer styles, same as for all of the Technics models.
Thanks also to many of you who have written to check that all is ok, I do my best to answer everyone but things have been difficult, all is improving now.
Good luck and best wishes,