If I have a large offline Subtangle with 1000 transactions and I choose to bring it online by attaching it to tip T, what happens if tip T ends up being invalid later on? Does my entire Subtangle become invalid? Do I have to reattach the entire thing to another tip or do I have to reattach all 1000 transactions to new tips?
Successfull merging of a subtangle and the main tangle: How to start a new offline subtangle?
The case, that you have to reattach your whole subtangle occurs when you start your subtangle at a transaction that is not very likely to be confirmed and conficting with the main tangle.
In this example, the two transactions marked with Xs are conflicting and the network/coordinator chose the white transaction to be the valid one. Therefore the red transaction and all transaction referencing it, also your subtangle, are invalid and have to be reattached at another transaction in the tangle. That means the PoW has to be done again.
When you have a large offline subtangle (which somebody already did the POW for); it is already attached to some (very old) transaction, which has hopefully confirmed until now (when starting an offline subtangle I'd start at a "confirmed" transaction anyway - at least as long as there is the coordinator, you can just use a milestone).
When you bring the tangle online, you can promote your subtangle's tips to get it confirmed. In case you attach the promote transactions at the wrong point, you'd only have to redo those, and not the whole subtangle.