I believe that the 1/2" copper is a direct replacement for 3/4" iron
pipe. 1/2" iron pipe is totally inadequate for any modern house. If
it were my house I would replace the larger runs with 1" or 3/4"
copper and the branch runs with 3/4" or 1/2" copper. Follow what
others said about the type of copper pipe

Paul J wrote:

I am ready and about to buy materials for the repiping from galvanized
1/2" to copper ?", but what size to use? I would like to go with 1/2"
type K but that takes up more inside diameter then M and L. Should I
go with 5/8" K, 3/4" K or just not be worried about the inside
diameter because it is a very small difference? Then again the
galvanized pipe probably has only an 1/8" openning anyway from all the
years of crap in it.