7 month old switching to formula

I just found out I am pregnant with baby #2 and am still EBF my 7 month old daughter. At her well baby visit this morning it showed a decrease in weight gain so they want me to transition her to formula as my milk is drying up now. They gave us samples of Gerber Good Start and we just purchased the Tomee Tippee closer to nature bottles. Any tips or advice on switching is greatly appreciated!