Is it possible to change the layout of a hood even when they're are Sims in it? I don't mean Sims already living there, just in the bin.

This is the only way I know of to change roads whatsoever in a neighborhood except using SimCity4: http://www.modthesims2.com/download.php?t=279992

There is no way to just draw roads where you want them in an existing neighborhood.

So if I used it I can still play Pleasantview? The only thing different is the roads?

If i edited a hood file from the sims 2 sc4 terrain files with SC4 would it work, or would it bork all hoods with that terrain?

A .sc4 terrain in the folder with all others is completely unrelated to any of your existing neighbourhoods. It's just based off the terrain, but it stands on its own and if the terrain is changed (and flattened) by placing lots, the original is, ofcourse, not.

To explain:
If you want to change the roads in an existing neighbourhood, you can do so by editing the terrain you based it off in SimCity 4, then creating another new neighbourhood with the edited terrain, and finally copying the new terrain over the old one by using Mootilda's HoodReplace programme, where HP linked to.

Why is then and some neighbourhoods created by Sim players using .sc4 terrains overwrite Pleasantview?

TIA

Go to your Neighbourhoods folder (Under My Documents > EA Games > The Sims 2 > Neighbourhoods (I think that's it)) and check your numbers. You see, Pleasantview is set as N001, but a custom neighbourhood might be set as N178 or N589. So the player who uploaded that neighbourhood probably deleted their Pleasantview (I know I did!) and then created the Neighbourhood, overwriting Pleasantview when players downloaded it.

If you want to keep both, you need to manually change the numbers around. Chances are that you've now completely lost Pleasantview and won't get it back (Good!) Unless you install a new Expansion, I believe.

First, thanks for the info, Vincent.
Secondly, losing Pleasantview is not good, because I have empty templates installed and there is no one in this neighbourhood but *my* Sims. So, no good in losing them.
Thirdly, when I deleted the said custom neighourhood, which seemed to have overwritten Pleasantview, the latter re-appeared again in my game.

Not sure if this is always the case though.