halving or doubling the value of notes to change tempo

I wrote a slow classical piece using Musescore (4:4 crotchet time) with a fast middle section. I realised only after I'd finished, that (rather than continuing the initial slow beat and writing semi-quavers, even demisemiquavers, as I had done) I should have written a new tempo of nearly double the speed for the middle section, which would have allowed me to halve the value of every note written (so that e.g. the semi-quavers become quavers), and more importantly so the beats are coming at right speed. But I can't find any way to do this, without re-writing the entire middle section which feels very cumbersome if there a quicker way using Musescore. (PS: I tried splitting the bars, which was easy, but then, for every 4:4 bar, I just got two bars with an unwanted 4-quavers time signature and it solved nothing - see attachment). Please advise! Thank you.