ive started a new characte rand i want him to become a good warrior, but later change and become a pure mage and nolonger use his sword.

has any1 done this effectively, or is ot not worth while focusing on say warrior to lvl 20 then do the rest as a mage.

Haven't tried it, but it might be difficult unless you forgo perks until you go mage. Or maybe save a few. And when you switch you might have trouble against level 20 enemies with low level mage spells you will only have access to.

EDIT: Maybe if you slowly transition to pure mage. Keep the armor and weapons handy but try to use mostly magic until you build up your skills

There are no classes. You can upgrade all skills to 100 in any order.

If you have problems upgrading your magic skills by hitting actual mobs - you can train on a summoned familiar.

the only problem might be the perks

No, but I have done the opposite, very effectively. I started Battle mage using only Destruction magic with light armor. At around level 30 I started using 1 handed, but kept destruction magic handy while early on in my levels.