Slightly faster would be to copy that block of cells down the column 4 times, then highlight the cells from Row 13 to row 24 and do CTRL-H (Find & Replace), and:

Find What: $M
Replace With: $N
Click Replace All

Repeat for the next block of 12 cells, but change the $N to $O in the Replace box, and so on.

You might want to change !B to !B$ first if you always want it to bring B21, then B22 etc.

Hope this helps.

Pete