wouldn't say rankings will be completely lost, but it would depend on your niche and how competitive your search terms are that you rank for, so a simply yes or no answer doesn't really fit your questions without some more details.
Generally rankings will decrease for a change of domains due to the new domain not having the links or authority etc that the old/current domain has, but if your not in a competitive market and/or have a relatively new site/domain to start with, your rankings shouldn't be effect to dramatically due to not having much to lose in the first place.
Any sort of change in domain or url has a risk of losing rankings though, so you need to gage if the change is worth the risk as you will loss link juice even with the correct 301 redirects in place and doing a change of address in google WMT.
So the question stands:
1. Why do you want to change domain names?
1. Is it worth the risk of having to basically start fresh?
Thankyou !!!
Best Regards .