Once we have wars in outer space, tanks will be obsolete

In outer space at what level? Just orbital? Then tanks will still be around because ground combat will actually still be relevent.
If you mean like intergalactic... well there will be a land element to those.

well, you still need tanks for their big guns.

Look at it this way, they have so many uses up to and including the warming of ones hands in the winter...... Now that.....will never be obsolete.... Seriously though, as long as there is action on the ground, you will need them.

Yes, you will need Tanks in one form or another. Maybe they will lose their gun and replace it with a missile launcher... who knows! But it'll still be a variation of a tank.
Only if you have those Battle Mechs running around will tanks be obsolete... and even in those games they have tanks because you can only afford so many gigantic robots!

they might get diffrent armor and weaponry, but they will remain...

Don't forget the fear factor also....when you hear one coming, whats the first thiing you do????? Start looking for cover.... At least until you figure out if it's yours or their's...and even if it's yours you may stay covered just so there is no friendly fire accident. That's one reasons Scouts used to be referred to as "Squishies" or "Crunchies"....either way its the sound you make as you get run over by a tank...