The term "station" and "depot" aren't quite the same--a "station" is a known point along the railroad tracks, a "depot" is a passenger terminal. The depot will not be moved--it will serve as the "grand entrance" to a much larger, expanded terminal and transit center. If you have ever been to Los Angeles' union station, it will be a bit like that--the grand entrance of the old depot is still the grand entrance, but once you go inside you take an underground concourse to reach Amtrak, regional heavy rail, Metra subways, buses of all sorts. The end of the concourse is a big bus plaza with several cafes and restaurants, with the Los Angeles Department of Transportation building and plaza at one end.
In Sacramento, in addition to two passageways under the tracks (one for bikes and pedestrians, one for baggage and transporters to carry those with limited mobility) there will also be an open transit plaza at ground level in between the tracks and the depot, with access to light rail, city and Amtrak buses, possibly Greyhound (the original plan was to relocate them to this transit center, the station on Richards is considered temporary) and maybe a new streetcar line. The rest of the transit plaza will be a bit like an airport concourse with restaurants and newsstands etc. on the way to and from the trains. The tunnels will also provide additional pedestrian connections to the Central Shops district at the southern edge of the Railyards proper.
The city already has money set aside, both its own and federal and state funds, to do most of this work, and a lot of it is well underway. If the city was to decide they want to do something else with the depot area other than expand the transit center, they would have to give a lot of that money back, including money that is already spent. (In fact, that's one problem with the arena plan: it would take over some of the space already allocated to the transit center, meaning the city would lose some of that transit funding, and have to reroute one of the underground passageways that they're actually building right now
, wasting a lot of time and effort. That's what happens when people let an idea advance too far without looking at what's already directly in front of them.)
While I like brainstorming and theorizing about what could be done as much as the next guy (assuming "the next guy" is a planning/transportation geek who spends a lot of time on Internet forums) turning plans into reality means connecting those plans into what's going on, and looking for opportunities to utilize existing momentum--or avoid existing resistance. Knowing the terrain, and building a plan to suit adapting to circumstances and specifications, how projects go from discussion to reality.