“It’s time we left” is in the past subjunctive, used to talk about unreal present (as in the 2nd conditional). We’re not leaving yet, so it’s unreal. Also, the unreality makes it seem more polite. Of course, “to be” is the only verb in English with a special form for the past subjunctive, so it’s not obvious to the casual observer. In American English, we often use the present subjunctive in your example, which has more of an immediate sense to it.

Luis, answering your second question: 1. “Change” has the broadest meaning. It can be used to describe almost any difference in a quality/situation/fact over time. 2. “Switch” is most commonly used when a person changes from one choice/tool/method/etc. to another distinctly different one. Example: “I switched from using a PC to using a Mac.” “Let’s switch. You do my job for a while and I will do yours.” 3. Shift, in contrast, can be used to describe a more subtle change — a change in degree, rather than a complete change in kind. Example: “Politics in that country are shifting to the right.” “He shifted in his chair (meaning, he moved his body slightly in the chair).”