In your screenshot I do not see any formula - are you creating a new one? If you are, then your inability to select a different Connection shows that the particular NPrinting App your report belongs to has only the one Connection.
Even if your App has more than one connection, once a formula is created you cannot edit it & change the Connection. You will need to Remove and re-create the formula. This is working as designed and applies to earlier versions of NPrinting as well.
HTH - Daniel.
2nd part of your answer applies to my query,I was editing formula and changing the connection, for already created formula.
It would have been easy, if there was designed, so we could have changed the connection, instead of removing and then adding it.It will be very hectic if we have more expressions or reusing npx file in different connection, with more number of expressions and images(i think this even applies for images and levels).