Copying/changing Local Time Signatures

I may not have understood correctly but...
Select eg. from measures 30 to end, press Ctrl+Del, save as *A.mscz;
Select measure 1:29, Ctrl+Del, select eg. from measures 60 to end, Ctrl+Del, save as *B.mscz;
And so on, could it be a workaround?

The answer depends on the version you are using and how you implemented the local time signatures.

In version 2, you can select a group of measures and use save selection to create a new file that contains only those measures. You do need to be careful to make the selection not be in the middle of a local time signature or you can expect the results to be bad. I have used this in large scores when I create them and also when I go back later to edit them. You should have no problems putting the score back together into a large file if you want using the Album feature unless you change the instruments.

Save Selection is not a real option in version 3 because it doesn't currently work well unless the score is in 4/4 (or C). Large score can normally be broken down into movements, but the only way I know to make this happen in version 3 is to select groups of measures that you don't want in the score and press ctrl+delete to get rid of them. You can then use Save as... to create a new file with the smaller score. The album feature is currently non existent in version 3, so putting the score back together later is not easy.

Seeing you score is the only way I can look at the local time signatures and give you more advice.