"Warning; this version is obolete;upgrade required!"
That's likely because of soft-fork changes that aren't supported by your current client.
My question is : Do i have to upgrade ?
You don't have to, but you should upgrade.
Or can i continue using old version safely ?
Yes, you can, but you shouldn't.
Lol, a lot of people is known if the new update is always having any improvement but maybe the op is still worried about upgrading bitcoin core into new version because worried to getting any bug or error in them.
No. There are no 'major bugs' AFAIK that should cause you trouble.