make 8th notes become 4th notes in a given MScore sheet

Hello,

I started to write a sheet that looks like this:

However, I thought it might be better to write the 8th notes as 4th notes, so it would look like this:

Is there a way to do this in mscore or do I have to type it all anew?
I tried saving the stuff as a midi file and then reimport it, saying the shortest note should be a 4th not an 8th, but then it looks like this:

which is not what I want.

Is there a way to change it without retyping all the stuff?

thank you

Hans