Does anyone here know whether Lancer Tactical's have easily upgradable gear-boxes?

If the unit is not an ambi-model, it's easier.

Other than that it's like any V2 in disassembly. So, overall...it's not easier or harder than say a split mechbox or a modular mechbox.

I'd just remove the ambi part anyway.

Is there a problem having a DSG with trigger contacts?

DSG does not affect trigger contacts...

No, but yes. You certainly can run a DSG with a traditional trigger switch but you'll need to pay attention to the cutoff lever interface. You'll also not be able to use AB to help prevent overspin in semi which could lead to jams etc.

Some Lancer gearboxes are OEMd by Lonex and are surprisingly well built, others are made of cheese and can bearly contain an m100. I'd be careful sticking a DSG in one without making sure the one you have is among the better ones.

The Lonex models are actually nice.

The V2 models I've been in have had some major issues and needed parts replaced that they really shouldn't have. For example, the two I opened both had nylon bushings that were slightly smaller than 8mm, so they couldn't be easily replaced without filing. They also had smaller than normal cylinders that caused extra drag and piston heads with nonexistent air seal.

The gearbox shells on the other hand... I have used mine with no radiusing aside from the weird stock radiusing with up to an M160 full stroke with no cracks. Mine may be the exception, and I only used the M160 for ~1,000 rounds, so don't take that as the rule. I currently have a Guarder SP130 in it...

I got a Lancer with the Lonex EBB shell. I disabled the EBB and run it as a DSG. Actually one of my favorite builds to play with.