I have a client who wants to change hosting.Right now he is using PLESK.
I will change DNS to point to my shared hosting (hostgator).

1-from where do i need to change DNS settings from within PLESK or domain registrar?
2-domain registrar and PLESK are two separate things (I know with CPANEL it is) the reason i ask is inside PLESK I see domain dns change settings,....
3-when i do domaintools i see Registrar:
Name: Tucows.com Co.
Number: 156

what this means?

thank you