I dug up the "System Requirements" websites for Lightroom. Here ya go:
https://helpx.adobe.com/lightroom/system-requirements.html
You'll see that LR 3 requires Mac OX 10.5 or 10.6, or Windows XP (SP 3) or Vista (various flavors); whereas LR 6 calls for Mac OS 10.9, 10.10, 10.11, and some 10.8, or Windows 7 (SP1), 8.1 or 10. First note that there are no os's common to LR 3 and LR 6. Second, I
know that a Mac running 10.5,6 cannot be upgraded to 10.9 or later; I
suspect that the analogous situation holds for windows. In other words, (LR3 -> LR 6) => (New Computer to run LR6). You may want to keep the old computer if you want to continue to use older (other) software for whatever reason. You'll have to examine the upgrade path for every software item you want to continue to use. PITA.
But wait! There's more! First, what I'm about to describe is a non-issue for Pentaxians, since newer Pentaxes have the option of choosing LR-readable DNG raw format right from the camera. For other brands, older versions of LR may not support RAW file formats for some newer cameras (in my case, a Lumix ZS50). Check whether the RAW format of your NEW camera can be handled by LR3.5 Then, you may find you need to use a recent version of Adobe's (free) DNG converter to convert your RAW files into LR3.5 -readable DNG's. So, if you want to continue with LR3.5, be prepared to download the converter so that when necessary, you can do a standalone conversion of the camera's raw files to DNG. I do this routinely as part of my work flow, and it isn't too bad. I will NOT be assimilated, I will NOT be assimilated...
This whole thing reminds me of Catch-22. Yes, it is possible to update Lightroom from 3.5 to 6, but there's a catch....
Best of luck - Gerry
---------- Post added 04-29-16 at 08:31 AM ----------
Stevebrot,
Did you upgrade from 3.5 to 5 on the same computer? If you succeeded without changing your OS, then Adobe's info is incorrect and their OS requirements are unduly restrictive. On the other hand, if you had to change OS, your experience might be helpful to grispie. Me, I'm really curious.
Thanks