They're only for current subscribers, and there's no way to go back and get past rewards once you've subbed.  Some details on what Preferred players get: [this Support article](http://www.swtor.com/support/helpcenter/6817) and [the comparison grid](http://www.swtor.com/free/features).

Note that this is separate from the credit escrow — any credits you gain while you have more than 200k (as F2P) are given back to you if you sub.