Yes, you must remove the old population before you can get the benefit from the new, breathable population. Personally, I just space the old population. But I'm a rather evil ruler
As for battleships, they allow more spaces for armor and/or shields which in are much, much better than LC's in the end. But wait until you have good shields before going for Battleships, because their extra room for shielding is, IMHO, one of their best attributes. The heavy mount does more damage per kiloton of space used, so yes it's an advantage over the LC's large mount, but takes up more space per component which renders it more likely to be destroyed first in the event of a battle. Wait on Battleships until you get more important techs, like full PPB's, ECM, and combat sensors. Stellar harnessing tech too.
__________________
Courage doesn't always roar. Sometimes courage is that little voice at the end of the day that says "I'll try again tomorrow".
Maturity is knowing you were an idiot in the past. Wisdom is knowing that you'll be an idiot in the future.
Download the
Nosral Confederacy (a shipset based upon the Phong) and the
Tyrellian Imperium, an organic looking shipset I created! (The Nosral are the better of the two [img]/threads/images/Graemlins/Grin.gif[/img] )