Sign up now, generally speaking any sign up bonuses will be negligble at best. Plsu you get access to BA's oneworld availability tool which shows reward inventory for all partner airlines.

Transfering your miles means simply shifting miles from one account to another. For example, transferring 100K MR points to 100K DL miles. Redeeming miles means spending miles on one program for flights on another carrier- ie. spending 100K DL miles for flight(s) on KLM or China Southern.