How do I change the facebook account I'm connected with on the prarty boat?

Go to options, go to manage accounts (which is the bottom option on the right), log out of the FB account there and log in with the different one. You might have to log out of the FB app on your phone, if you have it. See if this works.

Yes! I can confirm this works. Thank you!
1) Log out of Facebook on Sims Freeplay.
2) Log out of Facebook on your device.
3) Go back into Sims Freeplay.
4) Select the Facebook icon in Sims Freeplay.
5) Select "Log On". You'll be promted to enter your email/phone number and password. This will also log Facebook back in on your device.
6) Go to "The Party Boat".
7) Select "Add Friend" and viola!

I want to change the Facebook account but I don't want to lose the data, I don't want to start from lvl 1 again. What can I do in this case?

I'm very curious about something that happened in my Sims today: I changed my account of Facebook in my mobile to other account and suddenly new neighbours appeared, but also my own account! So I basically visited myself and even Woo Hoo with the same character I was using to visit the town. Then I added some Objects I needed to fulfil tasks and successfully returned after "saving Changes in the cloud" and the Items were now there, so I could use them. Everything was perfect, and since I have an iPhone with the same account but different game I tried and It worked as well, but it would send me to the town on the iPad. Everything was joy and perfection... BUT suddenly I don't know why It stopped showing me myself as a neighbour and automatically used the old Facebook account. On the other hand, my SIM on the iPas was still performing a task in the "other dimension" town even though I does not longer appear in the list of neighbours. I discovered that if I log out the Facebook account and use "another sim town" option it takes me there and now the time is frozen (I have a baby that I grew into a toddler that is still a baby in the "neighbour other dimension" and there are sims there that are not longer in the house. But that is not everything, if I come back to my town I can see my sim doing the tasks as if it was there, and if it ends the tasks nothing happens, but then I go the the "neighbours other dimension " and it gives me the completed tasks and points !!!!
My question would be, how can I go back to the star where I could neighbour myself and perpetuate that?

Assuming you're on an apple device, go into the settings and scroll down until you see the Facebook section (below twitter and above flickr). Go into the facebook section and delete the current account - then sign into your account. Go back to the Sims and try logging in again! Hope this helps :-)