The sign will be removed temporarily, it may be an entirely new sign in its place, but there will be sign here. All this is doing is eliminating the the poles and half wall on the right hand side to widen the walkways.
I can't find it right now, but Disney did mention this on the parks blog.
And technically they do have to "demolish" the existing sign in order to accomplish this, which is why the permits had to be filed.
In short:
- Adventureland will get a much needed wider entrance/exit
- The sign will be removed during construction
- The same sign (or a similar sign) will be put back in place before the construction is finished
So, no need to worry, it will still have the same feel, just a refreshed and easier to use entrance and exit.