Anyone replaced the cast front sight base with a forged mil-spec fsb?? I don`t have the forged fsb yet nor have I removed the Dpms fsb. I was wondering if the holes on the new forged sight will match-up with the barrel notches on the Dpms M4 barrel.

Not sure, but I thought the sight tower on DPMS's were forged.  
Look at the "up" lettering on the tower. If it's casted, the letters will be raised, and not punched that are used on the forged units.

As for swapping towers out, the tower is drilled after it's placed on the barrel. Because of this, you will need to drill new holes to mate the two, Hence the tower will come with no holes drilled.

Dpms Fsb is cast, albeit it looks good I bought a forged take-off on e-bay so it`s drilled for pins already. Hmmm... I might just leave the cast fsb alone unless I break it(I doubt it). I just assumed the holes would line up no problem.

The holes on your used unit might line up, its just not guaranteed.