I have been using Eagle since ver. 3.x. The first version I bought was 4.03 (©2001). It even came with a nice hard-copy instruction manual of just 208 pages.
So far as I know
libraries are compatible between the various purchased or free versions. That is to confirm what spec said that the limitations are on size, layers, sheets and so forth. To be clear, a project done with a free or other limited version can be accessed by a higher version of software. The reverse may not be true, as it is likely to include elements such as size, layers, sheets that the more limited software is restricted from doing. Whether one could produce a device with a pro version that would not be compatible with a free version, I don't know. I suspect that if one really tried hard to do that with a pro version, like make a device that is huge, it could be done.
There is an issue of serious compatibility between versions <6.0 and versions 6.0 and later (I think that is where the change occurred, but am not certain). Later versions of the software can open earlier versions, but once saved from the later version, the earlier version will not be able to open it again. I am not sure how that limitation affects libraries. It may, as I believe the change had to do with file structure.
Mikebits: That latter limitation is something you need to consider. If you open something you made with pro 5.0 using a free version after 6.0 and save it, you may not be able to open again with 5.0. So, be sure you have back-up copies made and saved with 5.0.
Importing Gerbers in a commercial environment might be a nice feature. It is just of no interest to me.
John