... a 'sandbox' is a closed, private system - seperate from whatever else may be happening, possibly taking place at a different time and even in a different world/universe. I think you want 'open-ended', not 'sandboxy'. You want your players to find the world to be available to them, to go on tangents, to follow up quirky leads, to MAKE quirky leads, get involved with their Contacts' lives, etc. etc. Open-ended, not sandbox - a box, after all, is closed.
And open-ended is what Shadowrun really is about. If your GMs ran it very tight ... you missed out on a lot of the game's possiilities.
That said, I think you should look to set your game somewhere where Lone Star still has the contract, and Knight Errant is the dedicated team; LS being lazy and corrupt is, after all, their reputation, and KE is reputed to be above reproach, strives for excellence, etc. etc. Set the game in an earlier Seattle, or just give the contract back to the Star so you can have that difference in effect.
Beyond that ... I'm not sure I can give much more advice than everyone else already has, or that you've already planned for. Be flexible; be ready to pick up the weird stuff your players talk about and run with it. Since you want a more loose campaign, your players are going to have to come up with their own runs - or else, if you run it smart, have a fixer as their close friend and confidante. I might suggest allowing them to pool their karma in order to purchase a single Fixer contact for the group, someone that'll be decently connected but be good friends with all of them - the NPC 'team member' that goes into the 'support team' instead of the 'front team'. Knows what they want, because they go drinking together, and when they lean in one direction ('We gotta find out what Kuato is doing in ZetaTech!!') can come up with something that's kind-of related ("I found a run for a guy who wants the plans for ZetaTech's new widget stolen') so that they can get access and do some of THEIR work while they're in there for the run.
Otherwise ... good luck.