Need an awk for a global find/replace in a file, specific column

I am new to unix and awk/sed etc... using C-Shell.

Basically, I have a fixed length file that has 4 different record types on it, H, D, V, W all in column 1. I need to change all the W's in column 1 to D's. in the entire file. The W's can be anywhere in the file and must remain in the same location as before.

Input File

H1111111111111
D1111111111111
D1111111111111
H2222222222222
D2222222222222
V3333333333333
W3333333333333
W3333333333333
V4444444444444
W4444444444444

The output file should look like the following:

Output File

H1111111111111
D1111111111111
D1111111111111
H2222222222222
D2222222222222
V3333333333333
D3333333333333
D3333333333333
V4444444444444
D4444444444444

Any help would be greatly appreciated.
Thanks.

 sed 's/^W/D/' file

Using awk:

awk '{if (substr($0,1,1) == "W") {print "D"substr($0,2)} else {print substr($0,1)}}' file

awk ' { sub("^W","D") } 1 ' file