Its bet to check with your health visitor first I remember when I did and when I told the health visitor she was'nt best pleased, if your changing from say farleys newborn to farleys for hungrier babies then all the green does is it takes longer to digest giving the baby the feeling that they are still full.
If you mean brands, then I am pretty sure it's ok ~ if you mean changing from newborn to 'hungrier baby' formula, or even follow-on formula, then it's best to check with the HV if you aren't sure.