I have been in touch with Abel Taiyo in Germany about upgrades, among other things.
For what it is worth, here is a summary of what I discovered:
1. Upgrades later than 1.4 are purely "cosmetic" in that they correct some language irregularities, in both German & English tables.
2. V 1.6 corrected some faults in V 1.5 and it not strictly necessary unless 1.5 had been installed.
3. Unless you are into semantics, V 1.4 is perfectly adequate and the upgrade is still available on two disks.
4. Finally, if anyone has installed Keysoft plug-ins, they should get their update files, if required, directly from Keysoft.
Mike O'R