The most bigger online wallets like xapo and blockchain recommend to change the address for each transaction for the security so it will be definitely good for us to change the address for each transaction otherwise they would not recommend that for us.

This.  My phone wallet generates a new address for every transaction, which I think is cool but kind of unnecessary.  I'm not doing anything illegal and thus don't need as much anonymity as some might.  You see the wallet address I have posted in my profile, and I'd be just as happy if that was the only one I ever had to use.

The funds will stay in your old address. You create the new address is to receive the money, not spend.

The best way to learn is to learn at your own pace. So take your time, hang around on this forum for like a month or two so you will know your way around in the world of cryptocurrencies.

I do not know about any harm for using new bitcoin addresses and I think it is much better for security as a number of exchanges and wallets are suggesting for to use new bitcoin addresses for each transactions. So it is for more security and people will not know about your wealth and will not try to hack your wallet.

I really don't like it, any people should have just one bitcoin address, I know a lot of people use it for anonymity but will be nice if someone create an altcoin that you have just one address and can't change it, it will avoid a lot of scams, bs and ponzi schemes.

Yes thats is the reason they provide and its not only Xapo but also blockchain.info that I use. It is of the opinion that when  you use just one address for all your transactions it makes it suspectible to hacking as a result of the value of transaction that will be carrying. So when an address is being used to receive Bitcoin, another one is generated for the next receipt but the disadvantage here is one for someone receiving small amounts get to pay more when sending payments because of the small amount spread in the account...

I think for most people, using a few addresses is fine.
You can set one for storage, and a couple that you use to send coins in and out of.

The main reason to use only one address would be a charity taking donations or something like that.
You would want to promote the address and not have to change it every time you posted on a site or document.

Pros: You will give anyone tracking you an incredibly hard time, and they'll be hard-pressed to get a good read on what you're doing with your wallet, so it basically gives you a security advantage.

Cons: It might be hard to organize, depending on the wallet layout. Aside from that I can't think of a ton of cons.