1) you are looking at updaters for fixing bugs found in earlier releases of the app. Keep looking, there's a free 30 day trial download that you can get for testing and evaluation purposes.
2) It may or may not be smooth. The change from 11 to 12 is a major design change that requires opening the FMP 11 files in FMP 12 in order to convert them into the new file format. Most of the time, that's all you need do, but there are some known issues that can be a bit of a hassle. Example, buttons on the converted file's layouts almost always suddenly shrink down to the minimum size needed to show the button text if you try to edit that button's text. You then have to resize the button to get it back the way you want. (As long as you don't try to modify the button, the buttons look and work just fine.)
If you are familiar with how to design layouts in FileMaker 11, doing the same in FileMaker 12 may drive you a bit crazy until you adjust to the changes as there have been a number of changes in how the selection box and alignment tools work.
And here's a link to a knowledgebase article on converting from 11 to 12:: http://help.filemaker.com/app/answers/detail/a_id/10571/kw/converting
Thanks for the information. I was more worried about the overall system work after upgrade. As I understand, everything should be OK, if I won't change anything after switching to 12?
Also, my company has an effective Annual Licence. Is it possible to register the program using the downloaded trial version? Or there is any specific procedure for annual license owners?
The last non-responded question is about installing FM Server. Do I need any special skills to do it?)
As I understand, everything should be OK, if I won't change anything after switching to 12?
As I said previously, it depends on your database. Using the trial copy to convert a copy of your files and testing them would be a good idea. What I posted was just one example of a known issue with converted files.
Is it possible to register the program using the downloaded trial version? Or there is any specific procedure for annual license owners?
As far as I know, you'll have to uninstall the trial copy and then install the licensed copy. I suggest contacting customer support for more about your options as an annual license holder.
about installing FM Server. Do I need any special skills to do it?)
I have never installed FileMaker 12. From my experience with an earlier version, I don't know if you really need any "special skills" or not. You do need to be familiar with the server's OS and how to work with the Firewall and port settings to get everything correctly configured. If you are publishing your database to the web, the added installation needs do make it a bigger challenge, but for just use over your local network, it's pretty straight forward as long as you have that basic knowledge about your server OS.