If anything i think it would be better to use the same agent to sell as well buy your new home, that way he or she can coordinate and time it so it transition go smooth. As an added bonus you can negotiate a lower commission also since out of you he or she will be getting 2 deals.
You're represented as the seller in 1 transaction & as the buyer in the other transaction. There is absolutely nothing wrong with the same agent helping on both transactions. You have someone looking out for your best interests both times. Plus, you get to know each other and the same agent is likely to be better at helping find you your ideal new home because they know you better than a new agent would. Why start all over again when you're already working with someone that should know your likes & dislikes?.
One big con on the other hand is if the areas are far apart, one agent might not be familiar with both markets. I would suggest using one agent if they know both areas. But have them refer you to another agent if they don't work much in the other market area.