This is what is recommended:
1. Hard reset your phone
2. Download developer preview app from the store
3. Install all updates
4. Hard reset your phone
Hard resetting will erase everything, but that is what has consistently provided best results. It may work without the hard reset, or it may not. Most of the problems reported on this site were due to skipping the hard reset.
IMHO if you don't want to do a hard reset, you shouldn't be downloading the developer preview.