"Also, what went down in Lion's Arch that made it appear to look like a war zone?"
- Long story short, a Sylvari mad scientist laid siege to it with an army of evil minions (y'know, the usual). Google "Scarlett Briar" and you'll get a summary of the whole ordeal from the GW2 Wiki. Lions Arch isn't far away from being restored (you'll see alot of construction work going on if you explore), at which time we'll be relearning the geography of the city all over again... again (Lions Arch seems to be the target of every invasion).
"Another thing I am confused about is how every time I am in the open world I get a message to switch to another board!?"
- This is GW2's attempt to create a "megaserver". Previously we were all playing on the Sanctum of Rall server, whether it be PvE or WvW. Sanctum of Rall only exists in name now for WvW. In PvE everyone is now combined into multiple mirrored maps that multiply or reduce depending on population. So for example, everyone who wants to travel to Queensdale all goes to the same map - when that map reaches capacity, a second Queensdale map is created, and all additional players end up there, then a third, forth, and so on. As people leave these maps and the population drops, you will get the message to "switch to another board" - this is Arenanet encouraging you to abandon the less populated map for a more populated one so they only have to keep open the bare minimum maps at a time (it also gives the impression that GW2 has a perpetually thriving population).
"I am scared to even go in WvW...I have read that has changed a great deal as well."
- Not as much as you'd think. The Eternal Battlegrounds map is the same as it ever was (and won't be changing when the Expansion comes along). The Borderlands maps are also pretty much the same (all the Keeps, Towers and Camps are still where you left them) they simply removed the giant lake in the centre, and filled the area with more interesting geography for small scale warfare (and a few extra useful siege locations. The major changes are...
* Instead of capturing an orb for a bonus on the Borderland maps, there are now 5 capturable locations in the centre of the map. Hold 3 of these locations continually for a few minutes, and the bonus goes to your realm (across all WvW maps), and remains yours until one of the 2 opposing realms can hold 3 locations long enough to take it for themselves. Each location only needs one player to sit in it to capture (just as you would in a camp after the guards are gone), there are no guards to worry about, just other players.
* World Ranks - this is a separate levelling system for WvW (little blue bar at very top of screen). Each level gained gives you a WvW rank, and a point to spend in a list of bonus WvW abilities. Abilities range from carrying/dispensing supply more effectively, applying/resisting damage from WvW NPC guards/mercenaries, to making specific siege engines more effective. Just hit "B" and select the "Rank & Abilities" tab/icon (3rd one down) to see your points and abilities available. Choose carefully though, as these are only reset at the start of each WvW season (though presumably a new season can't be too far away).
Hope this helps.