Building a new site in sub directory then replacing the current site

hi all, quick question, I’ve made a new instillation of wordpress at sussexchef.com/dev and I’m about to start building pages, obvoisly I’m going to move it to sussexchef.com when its all looking right. when I choose my page address links/ permalinks thingy, should I use new url names that don’t exist already on the old site? or should I keep the old url names, but include the “dev/” in the url name? But then will I have to go through almost 100 pages removing the dev/ and also changing all the links too? This would be days of work!
So confused!
please help,