Solebon is the paid-for version of SolFree by smallware. I cannot get in contact with smallware and the current owners of the modern versions of Solebon don't have any information or data related to the Palm OS version (and they are a different company, sharing only the name).
If you represent smallware, please make sure to make yourself known so we can remove the patched version if you want it taken down, or replace it with a proper version if you're willing (and able) to contribute that.
If you have an old version of Solebon, the upgrade files will detect that and work.
If you don't, install the Solebon_15_Upgrade.patched.prc, which is the same file as the v1.5 update, but with a single byte change - instead of giving an error when it doesn't find the old version, it just continues. I've tested this on a number of devices and pretty much all of them seem to run it properly.