Warranty work should be invoiced to BMW so in theory no problems. Maybe don't hold your breath at being first in the queue with a local dealer (me being cynical).
Just tell your local dealer you'll be buying your next car from them.
I was offered any warranty and servicing work would be carried out by a rival dealer despite the fact I didn't buy the E92 from them but had previously thrashed their demo model. They don't know when you'll be back.