It's not strictly true that you can't take the spray mechanism top off and not put it back on. That all depends on whether the spray mechanism and the surrounding material is made of metal or not and on how new and recent your bottle's spray mechanism is. Let me explain. It's a little involved.
You'll notice when you take the neck band off if the entire spray mechanism is made of metal or not. The neck band, suprisingly enough, is the round band that slips over the spray mechanism and hides from view where the spray mechanism connects and is crimped around the neck of the glass bottle. If you take the neck band off, which can be a real bitch, but with a little persistence and the right tools can easily be done, you'll will notice whether or not the entire spray mechanism is made of metal, and if it is made of metal that the metal is crimped around the class bottle. At this point, it is advisable not to try to remove the spray mechanism. First, because it is a major undertaking and, second, because if you remove the spray mechanism, you will depressurize the contents. There's a reason it's crimped, and that is because it is not what you call a natural spray, in which case the whole mechanism can be screwed off or pulled off and which operates by pumping air in when you depress the sprayer so that the air can push the contents of the bottle out. A crimped metal spray mechanism means the bottle is pressurized, which in turn means it operates on the principle that every time you depress the sprayer, you release some of the pressure in the bottle (through a value system) and the juice flows out with the pressurized air.
Now, if you remove the neck band and see that the spray mechanism part that connects around the bottle's neck is plastic, you will realize that the neck band is actually acting as the crimping force and keeps the whole system air tight and means the bottle is not a pressurized one, but a natural spray. In such a case, it's not a screw-on natural spray, like the Gendarme bottles, but a spray mechanism held in place not by crimping as in the case of a entirely metal spray mechanism, but by a plastic spray mechanism that is wedged and held in place by the neck band. In such a case, when you remove the neck band, you will find it quite easy to remove the entire plastic spray mechanism, and this will enable you to add or decant juice. You can then simply click the plastic spray mechanism back over the bottle's neck and secure it with the neck band, which will hold it in place once again. Gucci Envy, Tuscany, Havana, are just a few examples of such a plastic spray mechanism that can be removed and put back together again without any problems.
It's easy to remove the entire spray mechanism once you've removed the neck band and it's just as easy to put it back and secure it again with the said neck band. Since such bottles are natural sprays instead of pressurized sprays, you will be able to spray once again when you put the whole thing back together after taking it apart.
One will find that many modern bottles use this plastic spray mechanism that is secured around the neck of the bottle with the neck band. In all such cases, you should be able to remove the neck and spray mechanism, add or decant juice, put the whole thing back together again, and spray away as if you never touched the bottle in the first place.
I hope this all makes sense.