Informatica 7.1.5:How to replace the old connection with a new one

How to replace the existing connection with a new
one without redefining all source/target/lookup connections in every
session?

All connections whichsover you will create will be stored into INfa meta.And thus any any changes in connection will make sessions invalid.
In that case after creating your new relational connection,replace the old one with new one.You can do that from the menu CONNECTION.