This simple code that simply tries to replace semicolons (at i-specified postions) by colons does not work:
for i in range(0,len(line)): if (line[i]==\"
If you want to replace a single semicolon:
for i in range(0,len(line)): if (line[i]==";"): line = line[:i] + ":" + line[i+1:]
Havent tested it though.