Duplicating maps and monsters is easy, but between pre-renewal and renewal you're talking different game mechanics, that would require major source code modifications to have those different mechanics on the same server. At that point it's easier to have 3 servers under the same name/brand. Like some servers have both a mid and high rate that players can access.
Squishing just classic and pre-re together might be easier. You could even have the old maps, like Morroc not destroyed for classic.
However, what is the net gain in having all on one server? Shared @main chat and pms? A higher player count maybe. Then again, those players cannot really interact together.
The idea is interesting nonetheless, who knows maybe someone will read this and give it a try.
To me atleast and I'm sure there are others, get tired of servers offering only one era. Yes you can link servers together through the client, however you will still have to deal with starting all over from nothing which turns a lot of people off from changing servers after putting many months to years of their free time into it. And having to restart from nothing does lead to burnout among the player base leading to them taking long breaks or dropping the game all together, yes Im aware there are those that love fresh starts and server hop when a new one pops up that fits their rate liking. Plus the server host could put in some kind of character roll back npc/quest (back to state of entering selected era, classic being lvl1 novice or 99 second job, pre-renewal lvl1 high novice or 99 reborn). But this would allow players to continue their progress they have already built up into the next era if they wise.
Now to my limited understanding of the server file coding differences between pre-renewal and renewal, seems that the only change is the npc, db, mob settings/stats. Most servers now a days use the up to date kRO for all the client side bells and whistles,which you can edit the grf to add or remove maps, sprits, ect. As to why I mentioned above I am aware that it would take some time and effort to copy mobs and drop table as I know just leaving the basic setup in will cause double spawns as the user friendly text file you add extra changes too so renewal restricted items are not dropping on the earlier eras.
But I guess moral of the reason behind the idea is to bring more of the fanbase together on one server instead of different servers even if on the same clients listing. Plus more this is going to be a players home server and not home servers.