Question about changing lenses...

You don't need an L. Basically just use the lens needed for ths situation. Some situations might require more than one lens, others might not need more than one.

However, to address your comments. You don't need an L lens(although there are some fairly well priced L's[70-200 f/4L,17-40 f/4L, 200 f/2.8L]). There are many fine lens out there.

85 f/1.8 & 50 f/1.4 - prime like quality out of these 2, minus the price and build, but still great to have any bag. You can have both for between 600-700 USD, which ain't bad.

Sigma 24-70 f/2.8 & Tamron 28-75 - The midrange zoom is a very competitive one, and a great one to have a lot of lens that are of good quality here, since this range is often used for many normal things. Both are very well priced and deliever very good image quality. Better lens in my opinion than the 28-135 IS(unless you really need the reach).

Sigma 30 f/1.4 - Has had some quality control issues with some from the batch, but if you get a good one it gives you a great normal length lens on the 1.6 crop cameras.

There's a few to name, and many other threads about good quality lens. You can start here:
https://photography-on-the.net/forum/showthre​ad.php?t=56752

The more time you spend worrying about changing lenses the less time you will spend taking pictures.

Is it necessary to shut power down before changing a lens ? I do not....
But I read somewhere it may cause a short-circuit....

I always do although my understanding is Canon do not think this is needed. I have however, heard of people saying they have a camera lockup following a powered up lens change at least with some lenses. The suggestion seems to be it can cause message corruption between the lens and the camera perhaps with a low probability.

I have no idea if theis is the case or not.

I change lenses only once a year on January 1st. First, I shave off every hair on my body. Next, I shower with an abrasive soap to remove all dead skin cells. Then I seal myself in a temperature controlled bubble with an industrial-strength air filtration system. In fact, I guess I better get shaving.

Seriously though, I change whenever is necessary and often several times during an outing.

I change my lenses all of the time... and after every big trip (or when the sensor gets to dusty) send it to Canon and get them to clean it. I fedex it in they fedex it back, few days at the most - they are great.
I tried once to do it myself and made such a mess of it... leave it to the experts...

I change lenses whenever I need another one... I don't quite think I understand the question.

For most outings I try to plan ahead for the shots I intend to take. That's easy when I have a specific type of photography planned, like a sports event or a session sitting waiting for flying birds. In those circumstances I pretty much know beforehand what lenses will work. It's more difficult if I bring the camera "just in case" or "walkaround", in that case I usually take just the one lens on the camera and choose something generic like the 24-70 zoom, or just one odd prime like a fisheye for a challenge. Maybe a second lens in my jacket pocket. In any case I change lenses whenever and wherever I need another one for the shot.

Well, I don't know what your shooting style is, but I tend to be quite methodical about my photography. By that, I mean that I know what I'm going to shoot (or who), and what lenses I intend to use before I start. That helps me to minimize lens changes, as does having a couple of bodies. But, I also agree with Jon. Change lenses when you need to do so. They are tools and you shouldn't be afraid to use when you need to do so. Just learn how keep your camera clean (and how to clean your sensor), and enjoy shooting your photos.

Mark