You know I said about how one of them was wearing a cloak? That's Megatron. He's the one wearing a cloak. He's either going to be Megatron or one of Violenjiger's Destron Generals, and I'm fairly sure they weren't in Chicago to cause confusion.
So that's Starscream, all the original Autobots bar Bumblebee and presumably all the Protoforms in ROTF (unless I'm forgetting some other explanation for how they came to Earth from that ship) and a YMMV thing on Megatron and the Fallen's more elaborate spaceship modes and the fact there's no indication of any other way the first film Decepticons came to Earth either. I'd say that's enough of a representative sample to go with it being a standard ability, which makes sense considering their inbuilt alt mode changing abilities, all they really need to do is have the same space vehicle templates downloading.
How long have they been buried on the moon? How is that a big enough sample? I can name... what... thirty Pretenders. This does not mean all Transformers have Pretender shells. We do not know enough about the abilities of scanning alt modes, we can only infer. For all we know, being able to do it at a drop of a hat is a skill particular to Bumblebee, Skids, Mudflap and Megatron (who has the Allspark there). Indeed, the Twins' scene suggests some sort of preparation might well be necessary - otherwise they'd have scanned something, anything else in Shanghai.
There are far, far too many instances in the films where instant alt mode switching would be deployed - Demolishor and Sideways once their cover's blown, Team Sam when on the run, the Autobots when under aerial attack in Chicago, the Autobots when surrounded by military hardware in Egypt, Barricade when he's easily identified by Sam outside Mission City and so on and so forth - to suggest it's universally easy; Bumblebee's change is the freak occurence, not everything else.
All of which is rendered totally pointless by the actual intention of the Space Bridge. It's an instant attack, not a simple transport system (Charlotte Mearing actually uses words to that effect, with which Sentinel confers). Case in point - a hundred Decepticons appearing instantly in the middle of Washington DC, completely overwhelming the sizeable Autobot/human forces defending the area. Whereas a hundred meteors falling to Earth could be tracked and either shot down or have reception parties prepared for them in the time they take to fall.