Registering FL Studio or plugins while it is running is *not* possible. The program/plugins need to be closed and restarted in order to unlock them.
From FL Studio 9 onward you can:
Simply SAVE the project you are working on. The 'demo' version of the program or plugins will save correctly so you won't lose anything. You just can't open the project (as saved) UNTIL YOU REGISTER the program or plugin/s. After registration open the project and it will restore to the state it was when originally saved.
Don't open and re-save 'demo mode' saved projects, this will overwrite the demo data and you will lose it. You can re-open the project (let say you register a plugin and want to see if anything else was in demo that needs to be registered), just keep your finger away from the save button!
From FL Studio 8 and lower you can:
1. Swap out any plugins with ones that don't need registration such as 3OSC for synths and Fruity Reeverb for FX. Then rename the plugins to match the originals, so you know what to re-load next time. Of course you will also need to note what presets you were using.
2. Export your individual mixer track (stems) or the whole song out of the FL Studio demo version before closing the program. The audio export dialog
has a switch 'Split mixer tracks' this will save each mixer track to a separate wav file. Then, after registration, re-import these tracks into a new project as Audio Clips
(Producer Edition and higher) or Sampler Channels (Fruity Edition) and continue working on the project.
The FL Studio Support Team