Keyboard shortcuts for renaming file and accepting extension

You can disable the prompt under Finder's Preferences (see picture below):

This might save you some time if you are renaming multiple files. Additionally, you may want to look at Rename. It is a free utility that should serve you well for such simple batch renaming.

After trial-and-error, figured it out. It's ⌘+U.

Those buttons highlighted in blue (e.g in your case "keep old") press enter; for those button with a blue background (e.g in your case "Use ...") press space bar.

There is no keyboard equivelant that I am aware of...

You could however use the terminal to make your life a little easier. If you need to rename the files one at a time, use the command

mv orig_file_name.ext new_file_name.newext