question about new progression scheme

several achievement last for ever (e.g. reach level 60 with a class)

does that means that every season we will get this achievement automatically completed ? and get relevant progression reward?

I think achievements will remain completed, and they just add new ones.  (Some of) those new ones will award XP for the progression system.

Note that most achievements don't actually award XP, just 'Achievement Points' to indicate how many you have completed.