From: Near Portland, OR
I'm not familiar with any LCUs that leave and then return, but there might be.
With air units, if a unit withdraws out of theater, the pilots go with it. For air units that simply disband, the pilots go into the pool. If disbanded units reform, they usually do so drawing pilots from the pool, so some pilots may come back in the same unit by luck of the draw, but it's not hard and fast.
There are also some "named" pilots who are coded into the database from the start. Most of these are notable pilots from the real war. Most of these pilots are coded to go to a particular unit and if that unit gets withdrawn and returns, I believe they stay with it. Though I'm not 100% sure about that.
The actual mechanics of air groups withdraw/disband are pretty complex. We did what we could to make it easier for the user, but unfortunately there still is some complexity.
WitP AE - Test team lead, programmer