When you use a photo management program like Picasa or iPhoto or Photoshop Elements, there are basically three kinds of things you can do. First, you can edit the photo files themselves, for example, by cropping the image or fixing the contrast or color. Second, you can add "metadata" to the images, for example, captions and keywords. Third, you can organize your images into sets or groupings usually called albums. Photo management programs do many other things - make it easy for you to print pictures, upload images to a web site or email pictures to your friends, etc. But these three tasks - editing, captioning, and grouping - are the basic file management features shared by all of these programs.
Addendum: Perhaps I shouldn't call these "file management" features, as that suggests the kind of thing you normally do in your computer's desktop file management application, that is, the Windows Explorer or the Mac OS Finder. Perhaps I should call these "photo collection management" features.
In my last post, I noted that your image file edits get saved right in the files themselves. Ditto for the text captions and keywords, which, in Picasa at least, are saved as IPTC data inside the photo file. So, if you switch from one photo management program to another, you surely will not lose the edits to the picture itself and may not lose your captions and keywords.
You will, however, almost certainly lose your album groupings. An album is not a collection of files: that's what a folder is. An album a collection of file references, that is, of image references stored in the program's proprietary database. It's because albums consist only of file references that it's easy to put the same image in more than one album, without duplicating the image file. The point is that the information about these groupings is stored by each program in a proprietary database, that is, in a special file that can only be read by the program that created it. Now, if you spend tons of time grouping your images into albums, then you have an additional incentive to stick with one program through thick or thin. In fact, you don't have to change programs to lose your albums. With Picasa, at least, you will lose your album groupings if you move folders around, from one computer to another, from one hard disk to another, or even simply moving them around on the same hard disk. That's why it's best to import your photos into folders and then do all subsequent file management tasks inside Picasa. (If you're switching computers, you can preserve your albums by creating a special Picasa backup, using the backup command inside Picasa. When you copy the backup to your new computer, all your Picasa data - including album groupings - will be preserved.)
My solution to this problem is to create albums online rather than in my desktop software. The photos I have uploaded to Flickr and grouped as "sets" (albums) are actually stored on Flickr's servers and are thus indepenent of the different programs I used to upload those images originally. When I switched from iPhoto on the Mac to Picasa on the PC, it made no difference whatsoever to my photo sets on Flickr.com. But this is an imperfect solution at best. I am one of Flickr's early subscribers and I have a lot of photos stored there; but lately I've been using Google's Picasa Web Albums. What happens if I want to move my Flickr pictures over to Picasa Web Album's? I lose the Flickr set organization, plus any comments that were added to pictures in Flickr.
This is not a problem unique to photographs. It's a problem found in nearly all computer applications. OpenOffice.org will read your Microsoft Word documents (meaning you can abandon Word for OO.o fairly safely), but Word won't open OpenOffice documents (meaning you can't change your mind and switch back). Solution there? Don't use OO.o's native file format - always save your files as RTF, which is as close to a universal standard as word processing applications have. Want to abandon Outlook and starting doing email in Thunderbird? You're in luck: Thunderbird knows how to import your Outlook mail store. Want to move from Thunderbird to Outlook? You're out of luck. Outlook can import mbox files, which in theory are a common standard for email; but in practice, moving your messages from Thunderbird into Outlook is going to be a nightmare.
The solution to all of these programs is open, shared file standards. Every computer text processing program can open and read plain (ASCII) text files, but until every word processor stores document structures as xml, you're going to have to worry about whether you can read your old files when you move to a new program. Every photo management program and every computer operating system recognizes JPG and JPEG files now; PNG, TIF, and GIF formats are also pretty universally supported. But until all metadata about photos - including album groupings - is stored in xml, then moving from one program to another is going to involve not only pain but also loss.
Best solution? Keep printing out pictures. That way, you can enjoy them any time and you don't even need a computer.