Can a phone manufacturer's custom interface be changed? This is related to this question about removing apps, but I thought it was different enough to merit separate focus.

I recently bought a Samsung Galaxy 2 S, and it has a custom interface designed by Samsung.

Mostly, it's fine. However, some things bug me. For example, at the bottom of the screen, where there are standard buttons like "Phone" and "Internet" - things I'm likely to access very frequently - they have this stupid "SP Mode" mail app thing that I will never use. Ever.

I'd like to kill it, but after searching options, I don't know if I can make that kind of change.

Is it possible to get different "skins" and load them into the phone? Are existing interfaces modifiable?

Is this another thing that would require rooting the phone?
Your best bet would be to go with either LauncherPro: https://market.android.com/details?id=com.fede.launcher my personal favorite, or ADW: https://market.android.com/details?id=org.adw.launcher

Both provide customization to the dock (set of icons at the bottom), number of homescreens, transitions, app drawer effects, as well as custom themes and widgets. Note: there are free versions of both, but the paid versions add more features.

While you can cover up some of what Samsung has done, things like the Notification bar and lockscreen aren't affected by these apps. In some cases rooting and/or installing custom ROMs would be required to manipulate them.