I'll skip the obvious flaw in your statement, that you haven't found ANY software that won't run on it. I'll bet OS X won't ... ;-)
Most 128K will run on a 512K. Some will not recognize the additional RAM and load anyway running as if it were a 128K. Many will crash because of the presence of additional RAM, i.e. not expecting it. Also, most software that runs on and written for a 128K uses a different method than that written to take advantage of the greater 512K RAM. In particular, the 128K swaps out chunks of code which it reads and writes from the disk and calls from ROM, so that software is optimized for that process and may not work as well with more RAM.
I'm sure others have a better perspective on this. My main reason for wanting to switch between RAM is because I prefer to work within the limitations of the 128K, but recognize the need for 512K RAM from time to time, despite what I may have said above.
Most 128K will run on a 512K. Some will not recognize the additional RAM and load anyway running as if it were a 128K. Many will crash because of the presence of additional RAM, i.e. not expecting it. Also, most software that runs on and written for a 128K uses a different method than that written to take advantage of the greater 512K RAM. In particular, the 128K swaps out chunks of code which it reads and writes from the disk and calls from ROM, so that software is optimized for that process and may not work as well with more RAM.
I'm sure others have a better perspective on this. My main reason for wanting to switch between RAM is because I prefer to work within the limitations of the 128K, but recognize the need for 512K RAM from time to time, despite what I may have said above.