If the next full on expansion (say ship Interiors) is not free for LEP owners, then the LEP will have legitimate reasons to be angry (and all the legal stuff about breaking a contract etc, etc). However, I don't see them as doing this; Firstly the bad PR will alone would damage the brand badly, and they'd have a population drop like Odyssey. Secondly, you don't want to hack off your most loyal and enthusiastic players, normally, they're the ones that spend the most ARX, create the you tube videos and do (ahem)
podcasts.
Lets all be honest, we want Elite to keep running, and because Elite doesn't have the multiple platforms (anymore) that NMS has, Elite relies more on people buying ARX for its revenue that sales. So, it all depends on what you expect as an expansion. As long as those ships become available for in-game credits a few months later, then it's not excluding LEP players from the new ships. The 'Catch-Up' ship builds, if you're a LEP and haven't got ships which are light years ahead of them, then what have you been playing for last ten years?