Yes, there are players who move around and who fit outside the mold. This is more prevalent the younger players are and the lower the level they are playing at.
Also, most of the ratings that are going to be in these lower areas for most players will be the skill type ratings. Physical attributes will obviously be more non positional.
I don't really see a lot of QBs moving to LB but since that is the example you mentioned I will discuss that. I would agree that if you have a player who in high school played both sides of the ball or a player who is very athletic and strong or for whatever reason although he is a quarterback he has the physical ability to play linebacker.
I generally figure for instance that most quarterbacks should have tackle ratings, read play ratings, coverage ratings, etc... in the lowest area, 0 - 20 but if this guy has some experience using these other skills he might be up in the 30 - 39 or even the 40 - 49 area. If you move him over to LB while he is young then he will already have the physical abilities and with those skills being low with practice they will develop relatively quickly. If he moves to LB then skills he doesn't use should in my opinion atrophy as they are not being used.
This way players won't really be able to move back and forth constantly between several positions and the older they get the tougher it will be for them to change.
This way when a player is coming in to college where moving around is prevalent then a player who is versatile and really fits into another position could move over and be able to contribute pretty soon. With just the physical skills it might take a year or two.
However, if this player stays in his initial position through 3 or 4 or 5 years of college then you try to change them it will be more difficult, especially since they need to be at an even higher level to be successful, so it will be tougher to change their position.
Now if you leave them in their original position until they have been in the pros for 10 years it will be really tough to move them and they will probably only bring the physical abilities and any transferable abilities to the table.