Tear it out and don't rebuild, just read about all the fun everyone on here is having with their layout
If I was not happy with what I currently have, I'd tear it out and start over without another thought. BUT...I would put some track down and run trains for about a year BEFORE I made things too permanent by adding scenery and ballast.
I recently added a 2x14 section down the left hand side of my layout for a small yard. I've "re-designed" the track work probably 6 times since then and have had to scrape up ground cover every time. I found places in my track work that needed or could accept additional switches/turnouts, wider curves, longer/shorter sidings, etc.
An around the wall, shelf layout (I wouldn't go more than 30" deep, 24" will make the most out of a sheet of plywood) will give you more room and allow you to use larger radius curves. If you need to buy additional track there's always someone selling used track on the forum.