Could I revert system to Yosemite by using TimeMachine backup, after upgrading system to El Capitan? I'm considering upgrading system to El Capitan, but I have a lot of apps that could not work on new system. I have to check it by installing new system. I wonder if that apps would not work properly could I revert system to Yosemite by using TimeMachine backup, after upgrading system to El Capitan? Or it'll revert only files but on new system version? 

How does it work?

It's critical for me, because I'm working on this apps. If I wouldn't be able to run them I'm dead.
Yes, you can easily go back to a previous version of os x if you have correctly saved that version using time machine. 

You'll need to : 


connect the time machine to your mac
Restart your mac and hold Command + R
After the mac boots go to OS X Utilities and select Restore from a Time Machine Backup then hit next or continue
A screen will show Select a backup source select your time machine bacup
You will have next a screen with OS X Version and Backup Date and Time, If you want to go back to Yosemite select 10.10.X (Mavericks is 10.9.X)