Permanently... or the Rose Bowl...
My real question is: why is it worth it to build a whole stadium if there's already one available?
If you're just talking in terms of monetary cost, it likely isn't "worth" it. However, they aren't getting a team without a new stadium. And even considering the possibility of extensive renovations, it would be difficult to rebuild either stadium--both built in the early 1920s--to what an NFL team (and the league, of course, since they will have a say) would consider an appropriate level.
The Rose Bowl is probably the best site in terms of amenities, because it hosts the most individually-important bowl game--now often a championship game--and this has at least some corporate stuff like boxes. But the Coliseum is very centrally-located.
Either way, though, it would be a temporary site.