The formula should be the same makeup whether it is the concentrated liquid or the powder. My kids were the opposite. They had the powder but on a few occassions when we were traveling, it was necessary to use the concentrated liquid. There was never a problem switching back and forth as long as it was the same brand. If we had to switch brands for some reason, they would occassionally have a day or so where their bowels adjusted. They were usually on either the Walmart/Sam's Club brand or the Enfamil. If we switched to the Similac they would have problems with harder stools (but not quite constipation). If we switched to the Carnation Good Start, they would have much softer stools (but not diarhea). After a few days, they would even out some.
For the most part, all of the formulas are the same. There are a few minor differences but they are all FDA regulated and have to have the same basic makeup. With my daughter, after a few months, I switched her to the Walmart brand. With my son, we went with the Sam's Club brand. I checked the labels on all of them looking for the best for my child at the cheapest price. They were all comparable on what was in them. Price was another thing. The Sam's Club brand had much more formula in the can for about $10 less than the name brands at the cheaper Sam's Club price.
Good luck.