Different manufaturers for the generic drug.

I have been advised to replace amlodipine with diltiazem. Can I do it straightway? Or I should do it gradually?

yes

yes

It is a blood pressure medication. Diltiazem is in a group of drugs called calcium channel blockers. It works by slowing the electrical conduction in the heart, slowing heart rate, and/or normalizing heart rhythm. Diltiazem is used to treat hypertension (high blood pressure), angina (chest pain usually caused by lack of oxygen to the heart), and certain heart rhythm disorders).