I use to switch from MH to HPS after 4 weeks vegging. Now, I just stay with MH though the whole grow.

I also have better results with 20/4 in vegging. (18/6 in summer time)

...but, you can switch to HPS at any time. Most growers that switch; do it at about 2 weeks after going to 12/12 flowering cycle.

Some strains prefer MH. Some prefer HPS.