As sn0wxyuki said, TT'ing day by day is the safest albeit not the quickest method.
Walk in front of all your villagers to initiate a ping - to be on the safe side, I'd always check on my dreamies first.
You will probably need to save, quit and reload the game several times to catch everyone outside (in your town as villagers don't ping when they're on the main street, shops or at re-tail).
If you get no moving pings after doing this several times over, talk to your villagers as they sometimes mention if someone is moving (note: they will only do that if said villager is outside in the town).
If there's no mention of moving, you should be safe to TT to the next day and restart the process.
Sadly, there's no real way to keep any villagers "safe" per se. You may do this and find out that most of the villagers you want to keep are the ones that keep asking to leave. If that's the case, you just need to decline and repeat the process. It can actually take quite a few hours to get the ones you want out esp. if you have a large number of dreamies in your town.
Good luck and hope this helps.
