I reckon many posts have been made about this already, but I wanted to throw in mine as well, as well on how to easily fix commands that are affected by this change.

So, in the 1.11 Snapshots Mojang renamed many entities or put an underscore in the entity name. This ofcourse has its benefits but naturally also brings a lot of disadvantages to the table, the main one being that all commandblock contraptions that use entities won't work anymore in 1.11 if they use the 'old' name of an entity.
All the changes can be found here.

Now, to easily fix your commands for 1.11, the most obvious solution is ofcourse to manually replace all the old entity names with the new ones. However, for longer commands this might be too time-consuming. I found the easiest way to do this is to copy&paste the command into a text editor (such as Notepad) and use replace (usually Ctrl+H) (This thing).
Then write the names you want to change and then click on 'Replace everything' and you should be good to go!
Sorry for the different language, I might or might not have been too lazy to change my Notepad's language.

Also, keep in mind that if you change your commands so they work in 1.11, they won't work in 1.10 and below anymore as those versions ofcourse still use the old entity names.

Anyways, I hope I helped. If you have any question feel free to leave them and I'll try to answer them ASAP.