I recently installed Opera 21 on two machines, both with Opera 12.17 installed on them. I experimented a bit, and will also share my experiences:
I found the number of available Opera install files confusing, as I had been using Opera Next 22 on my desktop pc. On the other machine, a Netbook, I went to download the 'latest' Opera Next, and ended up with Opera 21 without realizing it. There are also downloads available with a "Campaign" tag in the file name. (E.g.: Opera_21.0.1432.67_Campaign_37_Setup.exe.) (More confusing, just to test, I went to http://www.opera.com/computer, and clicked on "Free Download for Windows" twice within a minute. The first time, I was presented with Opera_21.0.1432.67_Campaign_37_Setup.exe; the second time, with Opera_21.0.1432.67_Campaign_48_Setup.exe. I recall researching the 'Campaign' appendage yesterday, and it has something to do with Opera's 'user agent string', which is possibly being sent back to the company with crash reports, etc. Why they need it, I don't know, but must say I find the proliferation of install files, each differing by a few bytes in size, disconcerting! I finally was able to obtain Opera_21.0.1432.67_Setup.exe without any 'Campaign' designation, and used that for the install. [I don't recall now just where I found it, but I think it was on an Opera site, possibly an FTP site.])
After installing Opera 21 on a (64 bit Windows 7) Netbook, I went to my desktop pc and realized that Opera Next was at version 22, and that was the release version, not the 'developer' version for testing. I had Opera Next 22 on my (32 bit Windows 7) desktop installed in a separate directory in Program Files, and nothing was imported from v. 12.17. However, on the Netbook, I installed Opera 21 into the same folder as my 12.17 install, and the toolbar bookmarks were imported to the bookmarks toolbar in version 21. (Not, however, any of the other bookmarks from 12.17, not displayed on the toolbar. Those were not imported.) I also noted that Opera Next 22 had a white colored 'O' icon, whereas Opera 21 had a flattened red 'O' icon, making it very similar to Opera 12.17's rounded red 'O' icon.
Deciding to experiment a bit on my desktop, I uninstalled Opera Next 22, and reinstalled it in the same directory as Opera 12.17. It segregated itself nicely, installing itself into a sub-sub folder named with the version number, "22.0.1471.16". Version 12.17 was launched with Opera.exe, whereas version 22 was launched with Launcher.exe, which apparently 'launches' the Opera.exe file in the sub-sub folder under the version number. That way, both versions can coexist in "c:\Program Files\Opera" without conflict, as only the Opera.exe file for version 12.17 is in the "c:\Program Files\Opera" folder, the other Opera.exe file for Opera Next being buried one folder deeper. As I recall, now, Opera Next did not import any bookmarks from v. 12.17, nor did it create a Opera Notes.html file on the desktop when it installed. The icon to launch Opera Next was still the white 'O'.
Proceeding further, I next installed Opera 21 (without the 'Next' name, and no 'Campaign' number) in the same folder as v. 12.17 on my desktop pc. This time, I ended up with the red 'O' launcher (flattened to distinguish it from v. 12.17's rounded red 'O' icon). Similar to Opera Next 22, the install process created a folder named "21.0.1432.67" in the "c:\Program Files\Opera" folder, and it DELETED THE "22.0.1471.16" FOLDER, effectively uninstalling Opera Next v. 22! Also, with this install, my toolbar bookmarks were imported into v. 21 (those shown along the top of Opera 12.17's window, below the tabs). The myriad other bookmarks I have in v. 12.17, accessed in the Bookmarks sidebar that pulls out from the left, were not. Also, a file was created on my desktop called Opera Notes.html, as Notes are not (yet?) implemented within the v. 21 browser, and this file at least can be opened in a browser tab and read, so the Notes are not lost.
I finally installed the new Opera Mail on my desktop PC, and it pulled in all my emails, labels, and account settings from v. 12.17's integrated Mail client, with two glitches, one minor, one major. The minor glitch was that my old emails were being displayed as plain text, rather than as html, as in v. 12.17. This I corrected quickly by changing the display preference in the new stand alone mail program. The major glitch confused me for a bit, as I could not send nor receive any emails, despite the fact that my account settings had supposedly been imported properly from v. 12.17. I finally entered the passwords into the account settings, and the new mail program immediately worked properly, sending and receiving emails without a problem. The reason I was confused was that the settings window showed "[Password not displayed]" in the password fields, so I thought they had been imported, along with all the other info that was imported correctly. Apparently, they had not, and the new Opera Mail gave no error message of any type when I clicked on the icon to send/receive emails. It acted as if my mouse click had not been recognized.
I know the above is long, but I hope that it helps a few others who might be puzzling over the same issues as I puzzled over, while deciding which versions to install and exactly where to install them!