My baby eats similac isomil advance formula and I was thinking about switching him to nestle good start. It doesn't cost as much and I thought maybe it might help with him spitting up a lot to try a different brand. When we had his first doctors appointment 5 months ago they said it was OK to switch as long as it was soy to soy to see what he did best on. But now that he's 5 months is it still OK? And if its OK to switch how should I make the transition?

We did that exact change with our boy (now 2.5 years old). There will be an adjustment period in his system which may mean a bit of bad smell and odd diapers but we had no problem. He enjoyed the Good Start.

But how do I make the change do I just switch it just like that or do I have to wean him to the other formula

I honestly forget how we did it. I did search for other people doing the same thing and found this:
http://messageboards.ivillage.com/n/mb/message.asp?webtag=iv-prjul07n&msg=10313.1&ctx=0

Here they recommend mixing:
Parents Canada - Developing - Baby - Bottle Feeding With Lots of Cuddling (http://www.parentscanada.com/developing/baby/articles.aspx?listingid=72)