The thing is, I am in that position, I hadn't played sro in 9 years, and I joined this server about a month ago, the reason I was stuck here, was spending time with my friends which was nostalgia for me, but if you'd like to compare it to iSro I'm afraid it's not going to be that simple, for example the Academy was something that people loved because the concept of it changed the whole system, level up parties were expensive so people would join the Academy of a certain person so that they can get free lvls and the lvler can also get rewards, those are the things people most enjoyed, it would give you the feeling and sense of accomplishment, however, there isn't much of that in private servers, so I'd suggest the best thing is some really "emotional" clips of people killing uniques and the aftermath of it where people are celebrating, doesn't necessarily have to be real you can script one.
The other thing is fortress wars where also people had a lot of fun and memories in, you could also show the aftermath of that, how every person in thee guild would be proud with the badges by their names, walking around in the town that they've taken over.
And last, which is not that important of a suggestion, you could show specifically wings at roc mountain for nostalgia, as people spend months and years lvling up there, it'll bring back a lot of memories. pulling them back even if it's a little manipulative might be enough since they'll get used to changes and start to accept the new ideas of Private Sro.