The problem for bitcoin will be that "mining" for those old, "lost" coins will be more profitable than securing the network sooner or later. Imagine pools putting their miners on that task instead of securing the network. That will become a problem at some point
In the end you will never know if the early adopters are securing their coins or the black hat miners secured lost coins for them.
If a developper passes by, is it possible to enhance securities of the current bitcoins without breaking the protocole?
1st: I am a programmer
2nd: the security will be enhanced when necessary. This might get urgent tomorrow or in 100 years. 100 years might be the better guess but it's a guess. As we don't know how security will break, we can't just upgrade to a higher security now. Putting in more bits would be ridiculous and would have its drawbacks, too.