It depends on what your main symptoms are and why you are using Lansoprazole.
If it is only to reduce your feeling of heartburn and 15mg is working for you, fine. If it's not working, double up on your tablets: taking 2 x 15mg is the same as taking 1 x 30mg.
30mg is considered the high, attack, dose and 15mg the low, maintenance, dose.
However, if you have been diagnosed with Barrett's Esophagus, you may be best advised to remain on 30mg to quell silent reflux.
Barrett's cells are less sensitive to pain yet if they are still attacked by acid, there is the possibility this could cause the cellular change to pre-cancerous dysplasia.