@ pienari:
altium has apparently quite a lot of importers, but I do not like that company very much, and can't afford their software anyway.
It could be an option to get a free evaluation version of altium, import, and then import the altium projects in KiCad again.
KiCad does not directly import altium projects, but it does import altium schematics and altium PCB's.
KiCad can also import a set of Gerber files and convert them to a PCB, but as much info is missing in the gerber files the process requires a fair bit of additional manual work.
It is also quite possible there are online services that do this for a fee, but I never looked into that.
Also, your software is likely to be able to produce output of the schematic in some "universal" format, such as .pdf, .png and/or .svg, and when a project gets archived I tend to add such output to the archive, and also a set of Gerbers, because that is a pretty universal format too. With that information, any project can be re-created later. This is a quite labor intensive process, but it is relatively simple (and boring) data entry and that is much quicker then a complete re design. From some of my old projects (30+ years ago) a schematic in .png format is all that I still have left. But in the end it does not matter much. because, well, they are old projects. You may want to consider how much effort you want to put in converting your old projects. Maybe only 10 or so out of the 100 are important enough to convert and the rest can be archived and converted later only if the need to do so ever arrives. And who knows, someone may write a direct importer for pads to KiCad at some future date...