It's already possible to edit multiple tournaments at once. First select the tournaments you want to edit by either:
a) Using CTRL-A to select all of them.
b) A subset by holding shift and clicking the first and last of a group of tournaments.
c) Selecting multiple individual tournaments while holding the CTRL key.
When the tournaments are selected right-click and choose 'Edit Tournaments'.
With those tournaments loaded in the 'Tournaments Results' window you can again select tournaments as noted above and then edit more than one at the same time.
When saving a change to multiple tournaments at once you will be presented with this window:
So for example if you wanted to just change the rake for a bunch of tournaments but not change anything else you would edit the rake field only and when saving the change select 'Changed Settings'.