add domain to office 365

I need to ask whether is there anything will be change if I do the verification process for the domain? I mean the old configuration will be effected? Or nothing will happen?

Yes your old domain will be affected since you will be changing the MX, SPF, Lync etc records so that o365 can verify them. Depending upon your TTL settings in your DNS it may take a while 24-48 hours for the changes to be available on all DNS servers worldwide.

You can revert back (if you have a saved copy of the older settings) but you still have to live through the TTL wait.

I just have a domain name at an ISP somewhere with POP mailboxes and forwarding.
I have E3 plan.
I just need to verify my domains, nothing more, I’ll not change DNS or any other settings.
So still, if I’m doing the verification, anything will be changed in my old settings?

the verification process checks your mail settings.. ergo you have to change them.. see my original comment.

With E3 nothing changes automatically.

You create a TXT record in your DNS (which you can remove after verification)
MS checks the record is there - the TXT record is proof the domain belongs to you.
Then the domain is listed in Office365 as available for use.

It is only used by Office365 when you decide to change the DNS yourself.